INDUCTION. 596 INDUCTION COIL. any other material than that of the medium is iiilroduecd, llius making the surrounding medium lieterugcneous, Hutu will be iiidui-ed clmrgos on the body introduced. The question us to the cliaraeteV of the eharges on this body and their distribution dcpi-nds upon the relative elwlrieal induetivity of the body and the surrounding lue- diiini. (See Eucctkicity. ) Similarly, if there is a magnet surrounded by a uniforiii medium, sueh as air, and if a body of a dilf<Tciit kind of material from the nu'diuin is brought near the magnet, it will exiiibil magnetie forees and is said to be magnetized by induction. The charac- ter and distribution of" this induced magnetiza- tion depend upon the relative magnetic indue- tivity of the medium and the foreign body. (See Magnetlsm.) If the body which is introduced is iron or any magnetic body, and if the medium is air, the induced magnetization is such as to produce attraction by the magnet; if bismuth is introduced there will be repulsion. Electro- nuignetic induction is the plienomenon observed when tlu' magnclic lield of force iiicliidcd by a conducting circuit is altered in any way, viz. elec- tric currents are produced in this circuit. These induced currents are due to the changing of the field of magnetic force, and are in such a direc- tion as to tend to neutralize the change; they last only so long as the field is clianging. See Electricity. Tubes of Electric I.MircTlox are tubes which can be imagined drawn in the medium surround-
<X electrilied boilies by (tH«>sing any small closed eurve in this medium and drawing lines of force tlirough each point of it. A liollow tube is thus made which has one open end on a positively charged body and the other on one negatively charged. If' this tube is made of such a cross- section that it includes unit electrostatic charges as its two ends, it forms a 'Faraday tube.' TiBES OF Magnetic iNnrcTiox are tubes formed in the same manner in the field around magnets by drawing lines of magnetic force through the jwints of any small do.sed curve. Faradjiy conceived the idea of these tulies being continuous through magnets and all bodies, not ending on any surface: they form, therefore, closed circuits", like a rubber tube with the two open ends brought together. The tubes are con- ceived to be of such cross-sections that the num- ber leaving a north pole of unit strength is 47r, where IT =3.1410. See Magnetism. Owing to an electric current in a closed cir- cuit, there is a magnetic fiild of force inclosing it; the tubes of magnetic induction form closed curves around the conductor. The coe/ficicnt of self-induction, or the inductance, is the number of these tubes threading through the circuit when there is a unit current in the conductor. Some of these tubes of magnetic induction may also in their paths pass through a neigliboring closed conducting circuit, and the numt)er of those tidies which do so when there is a unit lurrent in the first circuit is called the coefficient of mutual in- duction of the two circuits. It may be shown that if there is a unit current in the second cir- cuit, thus producing tubes of magnetic induction of its own, the number of these which thread the first circuit is the same as in the last case. The coefficient of self-induction depends upon the fihape of the circuit, the number of turns of the conducting wire, and on the surrounding medium, and the coefficient of mutual induction depends upon these proiK'rties for each circuit and upon their relative positions. It can lie shown that inductance plays the same part in the phenom- ena of electric currents that inertia or mass docs in the motion of matter. The practical unit of induction is the henrj- (q.v.). INDUCTION BALANCE. An instrument to deteiiiiiiic lln- piVMiuc 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 alkiy. it was originally devised by Dove in 1841, but was improved and constructed in a serviceable form by Hughes in 187!t. It consists of two sets of induction coils in which the Jiri- maries are connected together in a circuit that includes a battery and a microphonic or other circuit-breaker, wl'iih' the secondaries are in con- 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 microplionc or circuit -breaker is not heard, owing to the inductive ell'ect being neutralized conipU'tely. 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 snuiid being tested by altering the jiosilions of the coils? An arrangement of the induction balance was devised by I'rof. .Mexander Craham Hell to locate a bullet in tlic liuman body, and ajiparatus based on the foregoing principle was employed nnd numerous dilfercnt forms made and tested. It was used in an attempt to discover the bullet by which President Garfield was killed, but the presence of the metallic mattress interfered with the operation of the instriunent. Instances, how- ever, of its successful use are on record. Con- .sult: Bell, "Induction Balance," in American .lounial of Science (New Haven, IBS,"!) ; Hopkins, Expcrimcitlal Kcicnce ( Xew York. 18!)0). INDUCTION COIL. A form of electrical ap- paratus used for transforming an interrupted current of low p<itential or pressure into one of high potential and alternating in direction. The fundamental fact of electro-magnetic induction was first stated by Michael Faraday in a paper presented to the Royal Society on November -24, 1831, in which he de"rived the conclusion that any ehan<.'e in a magnetic field will induce an elec- T _T FlO. 1. DIAOBAM OP INnOCTIOH COIL. trie current in a conductor situated in it. This basic principle of electro-magnetic induction underlies the construction and operation of the induction coil. As regards its construction, it consists of four essential parts: A bundle of soft
