INDUCTION BALANCE, in physics, an apparatus designed for measuring and indicat ing the electrical conductivity of metals. It is based upon the principle of induction. The mere proximity of a metal, especially a metal that is a good conductor, will induce or draw magnetization as electrification from a charged wire, battery, etc. An induced current is al ways opposite in positivity to the near side • of the charged source of current. In its most familiar form it consists essentially of four coils of fine wire, an electric battery, a circuit breaker and a telephone. Let the coils be desig nated, respectively, by the letters A, B, C and Di A being similar to B in all respects, and C being likewise similar to D. Coils A and B are placed in circuit with the battery, and coils C and D are placed in circuit with the tele phone. The coils are disposed in pairs, A be ing placed near C, and B near D. If the cur rent through A and B is rapidly interrupted, an induced current will, in general, flow through C and D, at each make and break; its presence being indicated by the sounds that it produces in the telephone that is in circuit with C and D. It is possible, however, to dispose the coils and their connections so that the current that A induces in C shall be sensibly equal and opposite to that which B induces in D. When the balance is perfect no sound
will be heard in the telephone. If a piece of metal be now brought near the coil A, the inter mittent current in A will induce currents in the mass of the piece of metal, and these, in turn, will act upon the coil C, producing in duced currents in this coil which are not com pensated by similar currents in D. The loss of balance will be at once indicated by the re currence of sounds in the telephone; and in this way the metal fragment will betray its presence. The induction balance has been used successfully for locating bullets in the human body. The intensity of the sounds produced in the telephone depends not only upon the size and position of the disturbing piece of metal, but also upon its specific electrical resistance; and hence the balance may be used to com pare the specific resistance of metals. When an induction balance is arranged for making measurements of electrical resistance it is usually termed an induction bridge.