INDUCTION BALANCE. An instrument to determine the presence or character of a piece of metal, which may be either concealed, as a bullet in a human body, or in the form of a counterfeit coin or alloy. It was originally devised by Dove in 1841, but was improved and constructed in a serviceable form by tlughes in 18711. It consists of two sets of induction coils in which the pri maries are connected together in a circuit that includes a battery and a microphonic or other circuit-breaker, while the secondaries are in eon nection with a telephone used by the observer. The current traverses the primary coils in op posite directions, and the secondary coils are so arranged that the sound of the microphone or pirellit is not heard, owing to the inductive effect being neutralized completely. If a coin or other metallic substance is introduced into the vicinity of one of the pairs of coils, this equilibrium will be disturbed, as part of the induction acts upon the metal and gives rise to induced currents. This of course produces an
audible sound in the telephone. The instrument is also used to measure hearing, and it then is known as an audiometer, the perception of sound being tested by altering the positions of the coils. An arrangement of the induction balance was devised by Prof. Alexander Graham Bell to locate a bullet in the human body, and apparatus based on the foregoing principle was employed and numerous different forms made and tested. It was used in an attempt, to discover the bullet by which President Cartield was killed, but the presence of the metallic mattress interfered with the operation of the instrument. Instances, how ever, of its successful use are on record. Con sult: Bell, "induction Balance,'' in American Journal of Science (New Haven, 1883) ; Hopkins, Experimental Science (New York, 1890).