Part 7 (1/2)
CHAPTER IX.
MINOR USES OF ELECTRICITY.
The electric ”trembling bell,” now in common use, was first invented by John Mirand in 1850. Figure 83 shows the scheme of the circuit, where
B is a small battery, say two or three ”dry” or Leclanche cells, joined by insulated wire to P, a press-b.u.t.ton or contact key, and G an electromagnetic gong or bell. On pressing the b.u.t.ton P, a spring contact is made, and the current flowing through the circuit strikes the bell. The action of the contact key will be understood from figure 84, where P is the press-b.u.t.ton removed to show the underlying mechanism, which is merely a metal spring A over a metal plate B. The spring is connected by wire to a pole of the battery, and the plate to a terminal or binding screw of the bell, or vice versa. When the b.u.t.ton P is pressed by the finger the spring is forced against the plate, the circuit is made, and the bell rings. On releasing the b.u.t.ton it springs back, the circuit is broken, and the bell stops.
Figure 85 shows the inner mechanism of the bell, which consists of a double-poled electromagnet M, having a soft iron armature A hinged on a straight spring or tongue S, with one end fixed, and the other resting against a screw contact T. The hammer H projects from the armature beside the edge of the gong E.
In pa.s.sing through the instrument the current proceeds from one terminal, say that on the right, by the wire W to the screw contact T, and thence by the spring S through the bobbins of the electromagnet to the other terminal. The electromagnet attracts the armature A, and the hammer H strikes the gong; but in the act the spring S is drawn from the contact T, and the circuit is broken. Consequently the electromagnet, no longer excited, lets the armature go, and the spring leaps back against the contact T, withdrawing the hammer from the gong. But the instrument is now as it was at first, the current again flows, and the hammer strikes the gong, only to fly back a second time. In this way, as long as the b.u.t.ton is pressed by the operator, the hammer will continue to tap the bell and give a ringing sound. Press-b.u.t.tons are of various patterns, and either affixed to the wall or inserted in the handle of an ordinary bell-pull, as shown in figure 86.
The ordinary electric bell actuated by a battery is liable to get out of order owing to the battery spending its force, or to the contacts becoming dirty. Magnetoelectric bells have, therefore, been introduced of late years. With these no battery or interrupting contacts are required, since the bell-pull or press- b.u.t.ton is made in the form of a small dynamo which generates the current when it is pulled or pushed. Figure 87 ill.u.s.trates a form of this apparatus, where M P is the bell-pull and B the bell, these being connected by a double wire W, to convey the current.
The bell-pull consists of a horseshoe magnet M, having a bobbin of insulated wire between its poles, and mounted on a spindle. When the key P is turned round by the hand, the bobbin moves in the magnetic field between the poles of the magnet, and the current thus generated circulates in the wires W, and pa.s.sing through an electromagnet under the bell, attracts its armature, and strikes the hammer on the bell. Of course the bell may be placed at any distance from the generator. In other types the current is generated and the bell rung by the act of pulling, as in a common house-bell.
Electric bells in large houses and hotels are usually fitted up with indicators, as shown in figure 88, which tell the room from which the call proceeds. They are serviceable as instantaneous signals, annunciators, and alarms in many different ways. An outbreak of fire can be announced by causing the undue rise of temperature to melt a piece of tallow or fusible metal, and thus release a weight, which tails on a press-b.u.t.ton, and closes the circuit of an electric bell. Or, the rising temperature may expand the mercury in a tube like that of a thermometer until it connects two platinum wires fused through the gla.s.s and in circuit with a bell. Some employ a curving bi-metallic spring to make the necessary contact. The spring is made by soldering strips of bra.s.s and iron back to back, and as these metals expand unequally when heated, the spring is deformed, and touches the contact which is connected in the circuit, thus permitting the current to ring the bell. A still better device, however, is a small box containing a thin metallic diaphragm, which expands with the heat, and sagging in the centre, touches a contact screw, thus completing the circuit, and allowing the current to pa.s.s.
These automatic or self-acting fire-alarms can, of course, be connected in the circuit of the ordinary street fire-alarms, which are usually worked by pulling a handle to make the necessary contact.
From what has been said, it will be easy to understand how the stealthy entrance of burglars into a house can be announced by an electric bell or warning lamp. If press-b.u.t.tons or contact-keys are placed on the sashes of the windows, the posts of the door, or the treads of the stair, so that when the window or door is opened, or the tread bends under the footstep, an electric circuit is closed, the alarm will be given. Of course, the connections need only be arranged when the device is wanted. Shops and offices can be guarded by making the current show a red light from a lamp hung in front of the premises, so that the night watchman can see it on his beat. This can readily be done by adjusting an electromagnet to drop a screen of red gla.s.s before the flame of the lamp. Safes and showcases forcibly opened can be made to signal the fact, and recently in the United States a thief was photographed by a flashlight kindled in this way, and afterwards captured through the likeness.
The level of water in cisterns and reservoirs, can be told in a similar manner by causing a float to rise with the water and make the required contact. The degree of frost in a conservatory can also be announced by means of the mercury ”thermostat,” already described, or some equivalent device. There are, indeed, many actual or possible applications of a similar kind.
The Ma.s.sey log is an instrument for telling the speed of a s.h.i.+p by the revolutions of a ”fly” as it is towed through the water, and by making the fly complete a circuit as it revolves the number of turns a second can be struck by a bell on board. In one form of the ”electric log,” the current is generated by the chemical action of zinc and copper plates attached to the log, and immersed in the sea water, and in others provided by a battery on the s.h.i.+p.
Captain M'Evoy has invented an alarm for torpedoes and torpedo boats, which is a veritable watchdog of the sea. It consists of an iron bell-jar inverted in the water, and moored at a depth below the agitation of the waves. In the upper part of the jar, where the pressure of the air keeps back the water, there is a delicate needle contact in circuit with a battery and an electric bell or lamp, as the case may be, on the sh.o.r.e. Waves of sound pa.s.sing through the water from the screw propeller of the torpedo, or, indeed, any s.h.i.+p, make and break the sensitive contact, and ring the bell or light the lamp. The apparatus is intended to alarm a fleet lying at anchor or a port in time of war.
Electricity has also been employed to register the movements of weatherc.o.c.ks and anemometers. A few years ago it was applied successfully to telegraph the course marked by a steering compa.s.s to the navigating officer on the bridge. This was done without impeding the motion of the compa.s.s card by causing an electric spark to jump from a light pointer on the card to a series of metal plates round the bowl of the compa.s.s, and actuate an electric alarm.
The ”Domestic Telegraph,” an American device, is a little dial apparatus by which a citizen can signal for a policeman, doctor, messenger, or carriage, as well as a fire engine, by the simple act of setting a hand on the dial.
Alexander Bain was the first to drive a clock with electricity instead of weights, by employing a pendulum having an iron bob, which was attracted to one side and the other by an electromagnet, but as its rate depends on the constancy of the current, which is not easy to maintain, the invention has not come into general use.
The ”b.u.t.terfly clock” of Lemoine, which we ill.u.s.trate in figure 89, is an improved type, in which the bob of soft iron P swings to and fro over the poles of a double electro magnet M in circuit with a battery and contact key. When the rate is too slow the key is closed, and a current pa.s.sing through the electromagnet pulls on the pendulum, thus correcting the clock. This is done by the ingenious device of Hipp, shown in figure 90, where M is the electromagnet, P the iron bob, from which projects a wire bearing a light vane B of mica in the shape of a b.u.t.terfly. As the bob swings the wire drags over the hump of the metal spring S, and when the bob is going too slowly the wire thrusts the spring into contact with another spring T below, thus closing the circuit, and sending a current through the magnet M, which attracts the bob and gives a fillip to the pendulum.
Local clocks controlled from a standard clock by electricity have been more successful in practice, and are employed in several towns--for example, Glasgow. Behind local dials are electromagnets which, by means of an armature working a frame and ratchet wheel, move the hands forward every minute or half-minute as the current is sent from the standard clock.
The electrical chronograph is an instrument for measuring minute intervals of time by means of a stylus tracing a line on a band of travelling paper or a revolving barrel of smoked gla.s.s. The current, by exciting an electromagnet, jerks the stylus, and the interval between two jerks is found from the length of the trace between them and the speed of the paper or smoked surface.
r.e.t.a.r.ded clocks are sometimes employed as electric meters for registering the consumption of electricity. In these the current to be measured flows through a coil beneath the bob of the pendulum, which is a magnet, and thus affects the rate. In other meters the current pa.s.ses through a species of galvanometer called an ampere meter, and controls a clockwork counter. In a third kind of meter the chemical effect of the current is brought into play-- that of Edison, for example, decomposing sulphate of copper, or more commonly of zinc.
The electric light is now used for signalling and advertising by night in a variety of ways. Incandescent lamps inside a translucent balloon, and their light controlled by a current key, as in a telegraph circuit, so as to give long and short flashes, according to the Morse code, are employed in the army. Signals at sea are also made by a set of red and white glow-lamps, which are combined according to the code in use. The powerful arc lamp is extremely useful as a ”search light,” especially on men of war and fortifications, and it has also been tried in signalling by projecting the beam on the clouds by way of a screen, and eclipsing it according to a given code.
In 1879, Professor Graham Bell, the inventor of the speaking telephone, and Mr Summer Tamter, brought out an ingenious apparatus called the photophone, by which music and speech were sent along a beam of light for several hundred yards. The action of the photophone is based on the peculiar fact observed in 1873 by Mr J E Mayhew, that the electrical resistance of crystalline selenium diminishes when a ray of light falls upon it. Figure 91 shows how Bell and Tamter utilised this property in the telephone.
A beam of sun or electric light, concentrated by a lens L, is reflected by a thin mirror M, and after traversing another lens L, travels to the parabolic reflector R, in the focus of which there is a selenium resistance in circuit with a battery S and two telephones T T'. Now, when a person speaks into the tube at the back of the mirror M, the light is caused to vibrate with the sounds, and a wavering beam falls on the selenium, changing its resistance to the current. The strength of the current is thus varied with the sonorous waves, and the words spoken by the transmitter are heard in the telephones by the receiver. The photophone is, however, more of a scientific toy than a practical instrument.
Becquerel, the French chemist, found that two plates of silver freshly coated with silver from a solution of chloride of silver and plunged into water, form a voltaic cell which is sensitive to light. This can be seen by connecting the plates through a galvanometer, and allowing a ray of light to fall upon them. Other combinations of the kind have been discovered, and Professor Minchin, the Irish physicist, has used one of these cells to measure the intensity of starlight.
The ”induction balance” of Professor Hughes is founded on the well-known fact that a current pa.s.sing in one wire can induce a sympathetic current in a neighbouring wire. The arrangement will be understood from figure 92, where P and P1 are two similar coils or bobbins of thick wire in circuit with a battery B and a microphone M, while S and S1 are two similar coils or bobbins of fine wire in circuit with a telephone T. It need hardly be said that when the microphone M is disturbed by a sound, the current in the primary coils P P1 will induce a corresponding current in the secondary coils S S1; but the coils S S1 are so wound that the induction of P on S neutralises the induction of P1 on S1; and no current pa.s.ses in the secondary circuit, hence no sound is heard in the telephone. When, however, this balance of induction is upset by bringing a piece of metal--say, a coin--near one or other of the coils S S1, a sound will be heard in the telephone.