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

latter by one primary coil can be neutralised by the discharged current due to the other primary, and the telephone will therefore give out no sound.

This is the arrangement of the Sonometer shown in Fig. 49, where a and c are the two coils of the

primary circuit connected to a battery and clockmicrophone; while b is the secondary coil in circuit with a telephone. Now the fluctuating current from the microphone traverses both primary coils a c, and a induces a current in the secondary b in an opposite direction to the current induced in it by c, and it is easy by shifting b between the primary coils to make these two induced currents equal as well as opposite.

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 B, 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

H

ing them to the same relative distances from each other. The induced currents in B D are then equal and opposite; the result being silence in the telephone.

This "induction balance" is, however, readily upset by introducing a coin or a metal wire into the hollow of one set of coils A B or C D; and Professor Chandler

Roberts has applied the balance in this way, not only to tell good coins from bad ones, but to assay the alloy of which the coin is made. This is done by putting a standard coin in one pan of the balance A B, and the coin to be tested in the other pan C D. Then, by means of a suitable adjusting scale devised by Professor Hughes, he was able to measure the number of degrees through which the balance had to be disturbed in order to bring back silence in the telephone. By repeated experiment he arrived at certain definite results: but the instrument proved really too sensitive, as it not only indi

cated differences in alloy but differences in hardness and molecular structure.

These and other experiments of Professor Hughes served to demonstrate the extraordinary delicacy of the balance as an instrument of research, and its sensitiveness to the presence of small metal masses when brought near to one or other pair of coils. Its practical application for detecting hidden metal masses, and playing, so to speak, the part of a divining rod, was suggested by the author, who, early in 1880, described an arrangement of the balance adapted to find out metal ores and veins in the ground. This arrangement, which indeed is the only obvious one, is substantially the same as that afterwards employed by Professor Graham Bell to locate the bullet

in the late President Garfield, and by Captain McEvoy in detecting the existence of metal-cased torpedoes, sunken iron hulls, lost cables, anchors, or other metal objects on the bottom of the

sea.

The arrangement will be understood from Fig. 51, where Ps and P's' are the four coils of the balance, arranged in pairs separated from each other and connected by insulated wires. The coils P and P' are joined together through a battery в and a key or interruptor 1, thus constituting the "primary" circuit of the balance. The coils s s1 are connected through a telephone T

FIG. 51.