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according to Fig. 47 where P is the stylus as before, tracing a screw-line on a revolving barrel wrapped with prepared paper and connected to the marking battery B, through the galvanometer G. The transmitter consists of a barrel H, enclosing a selenium cells, connected to the battery B' and to

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the stylus P. A fine pin-hole is drilled in the side of the barrel, which is revolved synchronously with the barrel of the receiver carrying the paper, and at every revolution the pin-hole crosses the surface of the cell in a spiral path. If now a luminous image of an object be focussed by the lens L on the barrel, just opposite the selenium cell, the pin-hole crossing the cell will let in a ray of light every time it crosses the image. In this way the pin-hole will trace out the shape of the image in parallel lines on the cell. But if the two batteries are so adjusted that when the ray of light falls on the cell the stylus will cease to mark, it

follows that the shape of the image will be delineated by the breaks in the parallel lines drawn on the paper by the stylus.

Fig. 48 represents a diamond pattern actually sent by the telephotograph of Mr. Bidwell. The luminous image focussed on the cell of the transmitter is seen on the left at A, and the picture of it outlined by breaks

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in the chemical lines drawn on the receiver shown on the right at B. With more delicate paper and a perfected apparatus Mr. Bidwell hopes to delineate light and shade in the image by fainter and deeper lines on the paper. The apparatus is one in its experimental state as yet, but the wires E w, Fig. 47, are intended to represent a telegraph line.

CHAPTER VII.

THE INDUCTION BALANCE.

THE most novel and fertile invention to which the use of the telephone and microphone have as yet given rise, is the Induction Balance of Professor Hughes. This apparatus has been employed as a sonometer or rather audiometer for measuring the hearing powers of the different ears, and as a means of assaying the quality of coins. Owing to its sensitiveness to the near presence of metals, a simple modification of it has also been applied to the detection of metal masses in its neighbourhood.

The Induction Balance consists of a primary and secondary circuit placed close together. The primary circuit includes a battery and a current interrupter, which may be an automatic key or a microphone placed on the base of a small alarm clock. The action of the key or the ticking of the clock vibrating the microphone varies the current in the primary circuit, and consequently the induced current in the secondary if sent through a telephone will produce a corresponding sound. The ticking of the clock for instance will be heard in the telephone. If, however, the primary circuit is doubled, and consists of two coils, one on either side of the secondary, the current induced in the

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When this is so there is absolute silence in the telephone, because there is a balance of induction. This position for b then is the zero of the sonometer scale, for there can then be no sound in the telephone, for any one to hear.

If, however, the middle coil b be displaced from the zero to either side the balance of induction will be disturbed, and a resultant current will be heard in the telephone. This effect will be louder the more the balance is upset, that is to say, the further b is moved from the zero position. It is easy therefore to test the sensitiveness of a person's ear to sound by first bringing the coil b to zero, then making the person listen in the telephone with that ear until he can just detect a sound in the instrument as the coil b is slid along the scale from zero. The further the coil has to be slid until the person can hear anything at all, the worse of course is his hearing for low sounds. In short, the number of degrees from zero on the scale is the figure of merit for his particular ear. Fig. 50 shows the form of balance employed in testing metals. Here there are two primary coils above, say a c and two secondaries below, say B D. The primary circuit includes a battery and clock-microphone as before, and also a small galvanoscope to show that a current is passing. The secondary circuit includes one or it may be two telephones. The secondary в is placed near the primary A, and D is near c, so that A can induce a current in в and c another in D. But D is reversed with respect to в, in order that the current induced in it by c shall be opposed to the current induced in в by A. These two opposed currents can be made equal by making the coils A B exactly equal to the coils c D and adjust

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