More Electrical Inventions

METALLIC FILAMENT LAMPS—THE FLAME ARC—MERCURY VAPOUR LAMPS—A STATIC CONVERTER—LIGHT-TRANSFORMING REFLECTOR—SILICA LAMPS—A ' WIRELESS' TRANSMITTER AND RECEIVER—CARDIOGRAPHIC APPARATUS—RAILWAY AUDIBLE CAB SIGNAL.

Electric Lamps

In the realm of Electric Lighting there have been several remarkable inventions since the beginning of the present century, but some of these do not call for much description. Indeed, so far as the metallic filament lamps are concerned, the chief interest lies in scientific discovery, and in the processes of manufacture.

The invention of metallic filament lamps consisted in substituting a metal for a carbon filament; but to do so was not an easy matter. The idea was older than the carbon filament lamp itself, as the first attempts at incandescent lighting were made with filaments of iron, platinum, and platinum-iridium. It was found that these metals had to run so near their melting-points that the carbon filament was far superior.

Recent scientific discovery showed how some of the rare metals, formerly only known in the chemists' laboratory, could be separated and worked on a practical scale. These metals are capable of withstanding very high temperatures. The melting-point of platinum is 2000° C., whereas one of these rare metals, tungsten, is as high as 3200° C.

At present our interest does not lie in the manufacture, but it may be noted that there are several methods of producing these metallic filaments. The first process was to make them from a paste, very much after the manner of the carbon filaments. The metal is reduced to a finely-divided state and made into a paste with gum, dextrine, or other binding material. This paste is then squirted through a very fine orifice in a diamond, under a pressure of several tons per square inch. The result is a fine moist thread, which is dried by heating in a vacuum, after which the binding agent has to be expelled. This is done by raising the filament to a very high temperature in an atmosphere of gases, whose chemical elements will unite with the elements of the binding material, and leave the pure metal intact. The fine metallic filament, which measures about one-thousandth of an inch, approximately as fine as a human hair, is placed in position in the vacuum glow-lamp.

In sonic cases the filament is made from pellets of the metal, and with modern wire-drawing machinery it is possible to draw the metal out to a filament of one-thousandth of an inch in diameter. Another method, which is used in connection with tungsten lamps, is to deposit the metal on fine carbon filament, by heating the latter in the presence of chloride or oxi-chloride of tungsten. The carbon is driven out, later, by heating the filament to a very high temperature in an atmosphere of damp hydrogen. These lamps have an efficiency of 1 watt per candle-power, whereas the nineteenth century carbon filament lamps consumed energy at the rate of from three to three and a half times that amount.

Another interesting invention in the same field is the 'Flame are,' or as it was called at first, 'Long-flame are lamp.' The electrodes in these have certain salts mixed with the carbon; the addition of the salts of fluoride, bromide, and iodide of lime give the light a yellow tint, while other salts of lime produce a red colour. Salts of calcium, magnesium, and strontium are used, and also the borates of soda and potash. The effects are very much more pleasing than the bluish-white glare of an ordinary arc lamp. But the chief value of the invention is not in providing a more pleasing light; the advantage lies in the production of an arc about five times as long as that produced between simple carbons. The flame arc lamps produce about twice as much light as the ordinary arc lamp for the same consumption of electrical energy.

Yet another outstanding invention in electric lighting is the mercury-vapour lamp. In 1901 a mercury-vapour lamp was shown as a curiosity at a social gathering at Columbia University (U.S.A.), and a few years later it became a practical success.

It may be mentioned in passing that mercury vapour was connected with our first knowledge of electric light, if we exclude the simple electric spark. More than two hundred years ago an experimenter happened to shake an imperfectly exhausted barometer tube, whereupon there appeared a luminous glow within the tube. It was soon recognised that this luminosity was due to friction between the particles, producing a state of electrification.

The Cooper-Hewitt mercury-vapour lamp consists of a vacuum tube with mercury for one of its electrodes, and iron for the other. When the current is passing through this tube, the mercury is vaporised and becomes incandescent. In order to start the lamp it is necessary to place the electrodes in contact, just as is the case in ordinary arc lamps. In the mercury-vapour lamp this is accomplished by tilting the lamp until the mercury runs over in a stream to the second electrode, making a temporary bridge for the current. The tilting of the lamp may be done automatically if desired. In this case, when the current is switched on, it passes through an electro-magnetic arrangement and back to the mains. In this short circuit the current passes through a solenoid, a cut-out, a substitutional resistance, and through a series resistance back to the mains. When this short circuit is energised the solenoid tilts the tube, and as soon as the current finds the parallel circuit through the stream of mercury it energizes an inductive resistance (in series with the tube), and this operates the cut-out. The tilting-solenoid being cut out, releases the lamp, which falls back to its normal position by gravity.

While this lamp gave a very economical light, consuming only one-half watt per candle-power, it had a distinct disadvantage in being devoid of red rays; a piece of scarlet cloth would appear black in its light. But in 1910 the inventor added a light-transforming reflector to compensate for this defect. This consists of a reflector, which diffuses the light, and the surface of the reflector is coated with a translucent material containing a fluorescent substance. The fluorescent substance has the effect of reducing the frequency of the waves, just as X-rays can be translated into visible light by means of a fluorescent screen. In this way some of the wave energy is stepped down into the red and orange portions of the spectrum, and this reflected light added to the direct light of the lamp produces quite natural colour effects.

The usual form of a Cooper-Hewitt lamp is a long tube measuring 18, 26, or 38 inches respectively. This is done in order to utilize voltages from 100 to 250 volts; but a shorter tube has been invented, and this may be enclosed in a globe similar to an ordinary arc lamp. This short mercury-vapour tube is made of quartz, and is known commercially as the Silica lamp. While quartz will stand a very much higher temperature than glass, the substitution of quartz for glass placed the manufacturer in a serious difficulty. He has no difficulty in conducting the electric current to the inside of a glass vacuum tube by means of platinum wires. Platinum and certain kinds of glass have practically the same coefficient of expansion for heat, so the seal will not break down when the lamp heats or cools. The expansion of quartz and platinum, however, are very different. The expansion of quartz is almost 0.4 micron per degree per unit length, whereas the expansion of platinum is 8.0 microns. (A micron is one-thousandth part of a millimetre, or one-millionth part of a metre.) With this ratio of 1 to 20 it is impossible to make a satisfactory seal; it is sure to break down almost at once.

This expansion difficulty was overcome by the invention of an alloy of steel and nickel, which the inventor, C. E. Guillaume, has christened 'invar.' This alloy expands 0.8 micron per degree per metre, and is thus very similar to that of quartz. Although invar gets over the expansion difficulty, it does not permit of a simple seal as is done with platinum, for invar is a forged metal which loses its properties when brought to red heat. To overcome this difficulty, a tapered rod of invar is ground into a conical quartz tube, and this mechanical seal is protected by a mercury cup, the mercury being retained by bitumen or cement.

Mercury lamps, being very rich in ultra-violet rays, have been used for sterilizing water, the rays being death to the bacteria. Another department into which the mercury vapour lamp is being introduced is that of electroculture. The radiations from the lamp stimulate vegetable growth.

A distinctly different use to which the lamp is put is that of a 'static converter,'' to convert an alternating electric current into a direct current. The lamp practically stops the negative impulses of an alternating current, and permits the passage of positive impulses only. When the lamp is used for this purpose it is provided with several positive electrodes, and the tube is made spherical in order to provide a larger area for dissipating the heat in the apparatus. Another advantage in the spherical tube is that the distance between the positive and negative electrodes is lessened, thus reducing the waste of current. By means of a mercury arc rectifier it is possible to charge accumulators from an alternating current circuit.

Wireless Inventions

As Wireless Telegraphy had scarcely gained a business footing until the beginning of the present century, it is natural that all the more important practical inventions connected with it fall within our present field of inquiry; but the subject is so wide and of such general interest that there is to be a special volume, in the present series, on Wireless Telegraphy. For our present purpose it will be sufficient to consider one wireless transmitter and one receiver.

From the simple spark-discharger, there was evolved in 1907 a large synchronous disc-discharger. This transmitter produces a series of tether waves, which set up a distinct musical note in the distant receiver. In this way the interference of atmospheric disturbances, or discharges from other stations, are distinguished easily from the particular signals transmitted from this synchronous transmitter.

When attempts were made to produce a musical note, using fixed electrodes in the discharger, it was found very unsatisfactory, owing to the formation of an arc between the electrodes. This difficulty was overcome by inserting between the electrodes a toothed wheel, which was rotated at a high speed. The sparks had to pass from one electrode to the insulated wheel, and from that to the second electrode. This prevented any continuous arc being set up.

We may picture the toothed wheel being replaced by a large insulated metal disc, with a number of studs projecting from it, so that the circle of studs will pass between the electrodes as the disc revolves. In place of two fixed electrodes we picture two smooth metal discs placed at right angles to the surface of the large disc, so that the revolving disc passes through between the two edges of the electrode discs, as shown in Figure 1.

FIG. 1: A TRANSMITTER IN WIRELESS TELEGRAPHY.

The dynamo charges the condenser D and the revolving discs B and C, and these discharge through the projecting studs on the large revolving disc A, which is insulated. This charges and discharges the condenser E through the inductance F, and these to and fro impulses induce a similar electrical disturbance in the aerial, which in turn passes the energy out into the ether of space, in the form of groups of 'wireless' waves. The smaller discs are kept revolving also. These electrode discs are connected to a high-tension generator, and the discharge takes place as described.

In the latest machines the number of studs on the disc are the same as the number of poles in the generator (alternator), thus giving one spark in every half-cycle. Prior to this synchronized arrangement, the disc studs passed the electrodes about five or six times in each half-cycle of the generator current, but the results were not nearly so satisfactory. We picture this wireless transmitter sending out groups of tether waves, which, arriving at the receiver, produce a clear musical note.

FIG. 2: A RECEIVER IN WIRELESS TELEGRAPHY.

A well-known form of receiver is the Marconi Magnetic Detector. Its general appearance is not unlike that of the telegraphone (page 53), but the two inventions have entirely different functions. In the magnetic detector there is an endless band of insulated iron wires, which are kept in motion by passing over two revolving pulleys (see Figure 2). A temporary state of magnetization is produced in that part of the iron band passing in close proximity to the two permanent magnets. The band being made of soft iron, there is no permanent magnetization in it. Indeed, the magnetic polarity of the different parts of the iron band will keep changing continuously as they approach and recede from the magnetic poles, and it is well known that this change requires time. The change in the iron will lag behind the application of the magnetic force, the phenomenon being known as 'hysteresis.' The change may be hastened by the influence of a neighbouring electric current, and this occurs in the following manner:

A wire leading from the aerial is wound around a small glass tube, through which the iron band passes at the moment when it is under the influence of the permanent magnets. This aerial wire is then led to earth. Any electric impulses in the aerial will hasten the magnetic changes in the iron. The electric impulses in the aerial will correspond with the electro-magnetic waves arriving through space from the distant wireless transmitter. The sudden changes of magnetism in the iron band will induce electric currents in any suitable surrounding coil of wire. Such a coil or bobbin of fine wire is placed, as shown in the diagram, around the magnetised portion of the band. This bobbin of wire is connected to a telephone receiver, in which a sound will be heard at each sudden change of magnetism in the moving band. And if the transmitter is sending out regular groups of waves of a suitable frequency, there will be heard a distinct musical sound.

The connection between the distant transmitter and the receiver is easily followed. By means of a special telegraph key the sending operator controls the electric impulses sent out by the transmitter. These impulses or waves pass out through the anther of space, and some of them are trapped by the aerial. The electric disturbance in the aerial wire affects the magnetic condition of the iron band, and the sudden magnetic changes induce momentary currents in the telephone circuit, and by the sounds in the telephone the Morse signals may be read.

Cardiographic Apparatus

The function of Electrocardiographic apparatus is to register the pulsations of the human heart. Experiments made upon the freshly extracted hearts of frogs and tortoises have shown that there exists a natural electric current, which varies as the auricle and ventricle of the heart contract in succession. The electric current is, of course, a very weak one, but its variations may be read by means of a string galvanometer.

The string galvanometer was invented by Professor Einthoven, of Leyden, and consists of a very fine silver-plated filament of quartz, measuring about one-twelve-thousandth part of an inch in diameter. The quartz thread is stretched at right angles to the lines of force between the poles of a powerful magnet. The incoming electric current on passing through the filament sets up a magnetic field around it, and the filament is influenced by the powerful magnetic field of the neighbouring electro-magnet; the middle of the conducting filament is deflected at each electric impulse. In order that the movements of the filament may be read, a greatly magnified image of it is projected upon a photographic plate or sensitized paper. By means of an arc lamp and a lens, light is sent into the dark-box through a lateral slit. The filament of the galvanometer is placed so that a shadow of it is projected upon the sensitized surface within the dark-box. Movement is given to the photographic plate or paper, and a record of the deflections of the filament is made.

At first it was thought necessary to place the terminals of the recording apparatus in contact with the patient's chest, one terminal over the apex and the other at the base of the heart. But it has been found that it is possible to conduct the current from the hands of the patient, who immerses his hands in two vessels filled with salt water and connected by conducting wires to the galvanometer. In the latest arrangement there is a third lead taken from the patient's left foot placed in a third vessel of salt water. A separate record is taken from each of these three leads. We may picture the human heart as a battery or a dynamo sending out impulses at regular intervals. These electric impulses are conducted to the filament of the galvanometer and thence to earth.

The apparatus consists of these water-vessels which act as terminals for the wires which have to lead the heart impulses to the sensitive string galvanometer. An arc lantern projects the shadow of the pulsating filament upon the photographic plate or paper-ribbon. If a sensitized glass plate is used, and this is the most convenient for general purposes, the plate is held in a compound slide, enabling any desired fraction of the plate to be exposed at a time. The plate, which measures 85 x 170 millimeters (3.25" x 6.5"), slides downward by gravity, and its speed may be adjusted to any value between 1 and 100 millimeters per second.

In the continuous-paper camera there is a reel of bromide paper about 75 meters long and 80 millimeters wide. It is driven by means of a small motor, and can be made to travel at speeds from 15 to 180 millimeters per second. The horizontal lines are photographically ruled at 1 millimetre intervals, while the vertical lines are obtained by means of a simple tetanus time marker, the intervals representing fiftieths of a second.

The transmission between the patient and the recording apparatus being electrical, it is possible to convey the impulses to a distance. For instance, Professor Einthoven had a recording apparatus in his laboratory connected by cable to the salt-water-vessels in the Leyden Hospital, about a mile distant.

Railway Audible Cab Signal

In the first year of the present century not a single fatal accident occurred to any passenger on British railways, but it has not been possible to maintain this record. Recent disasters seem to call for some automatic means of preventing an engine-driver running past a danger-signal, and so we are naturally interested in any reliable means of achieving that end. The Railway Audible Cab Signal has passed the experimental stage, and is in continual service on 1.10 miles of track on one of the principal English passenger main lines, also on a branch line, where it has replaced distant signals, and several other principal railway companies are seriously considering the adoption of this essential safeguard.

Instead of the driver and fireman having to keep a look-out for the distant semaphore signal, or its light, they may have the signal given in the cab of their locomotive. A dial placed in front of them beside their starting-lever and valve-handles, shows a white signal when the way is clear, and this is replaced by a red ground, on which the word 'Danger' is painted, when the way is not clear. There is a further safeguard to prevent the signal being overlooked. The moment the signal 'Danger' is given, a steam siren or whistle is sounded in the cab, and it will not stop until the driver lifts a lever. In order that the driver may know when he passes the distant 'All Right' signal, an electric gong is sounded, and continues to ring until the driver presses a push. It might seem quite unnecessary to add any further safeguard, but in addition the brakes may be applied automatically if the train should attempt to run past a danger signal.

At each place where it is desirable to give the driver an audible signal, a 'ramp' consisting of a bar of T iron, suitably mounted and insulated on a timber base, is fixed in the centre of the track. This ramp is connected by electric wires to the signal cabin, and if the way is clear the pulling of the 'All Right' signal-lever switches on an electric battery current to the ramp. From what follows it will be seen that the presence of this current causes the electric gong to sound and the white signal to remain visible in the locomotive cab. But when the signalman pulls his 'Danger' signal-lever, no current is sent to the ramp, and the siren sounds, the visible 'Danger' appears on the dial, and the brakes are applied.

It is clear that the locomotive must be put in electrical connection with the ramp, and this is done very easily by means of a sliding contact. The ramp, which is about sixty feet in length, is slightly arched, its two ends being at rail level and its centre rising four inches higher. The ramp is, of course, a fixture, and has no moving parts whatever. A shoe is fixed beneath the engine and in position to engage with the ramp. When passing over the ramp the shoe is raised one-and-a-half inches. The shoe is insulated from the mass of the locomotive, and in its normal position the shoe is held down by a spiral spring. When in its normal position the shoe closes an electric circuit on the locomotive, and a local battery current keeps an electro-magnet energised. This magnet holds up a lever which keeps the steam whistle closed, but when the shoe is raised by passing over the ramp, this electric circuit on the locomotive is broken, and the magnet lets go the lever, causing the whistle to sound and, at the same time, releasing the red 'Danger' signal on the dial.

This danger signal will be given every time a locomotive passes over a ramp, unless the signalman sends an electric current to the ramp, in order to prevent the danger signal being given; this is done automatically when he pulls the 'All Right' signal-lever. When the shoe picks up the electric current from the ramp the current energizes a polarized relay which completes another circuit to the electromagnet and holds up the lever, although the direct normal circuit to the electro-magnet has been broken by the raising of the shoe, and so the whistle is still kept closed and the visual 'Danger' signal is not released. At the same time, the electric current from the ramp operating the polarized relay also switches on the local battery current to the electric gong, which continues to ring until the circuit is broken by the driver pressing a push. It will be observed that, should anything go wrong with the signalman's connection to the ramp, or should he fail to give the 'All Right' signal, the engine-driver will receive the danger signal, for the shoe will be raised, breaking the local circuit on the locomotive and releasing the audible and visible danger signals, and simultaneously applying the brakes.

This system necessitates the provision of a continuous electric current on the locomotive to maintain the 'All Right' signal, but all that is required is a battery of four small dry cells. The cost is infinitesimal compared with the great safeguard provided. Should the current from these cells fail, the electromagnet will cease to be energised, and it will let go the lever causing the danger signal to be given.

In order to save the current while the locomotive is in the shed, there is an automatic arrangement by which the battery current is switched off whenever the steam in the locomotive boiler falls below 20 lb. pressure, the current being automatically switched on again when the steam rises to that pressure.

It requires no very lively imagination to appreciate the immense advantage in this audible cab signal in case of foggy or dirty weather, and it prevents any possibility of a driver omitting to observe a signal while he is attending to the mechanism of his engine. The safeguard is so great that the man-in-the-street will wonder that the use of such apparatus is not made absolutely compulsory.

During recent years there has been a host of inventions in connection with the automatic operation of railway points and signals, and it was the introduction of these which made it possible to accelerate the train service on the London Underground Railways. All this necessitates considerable expense, but the reduction in the number of signal-cabins is great, and in some cases one automatically-operated cabin replaces as many as thirteen manual cabins.