Electrical measurementsA laboratory manual . ulas. Fig. 138. 145. The Ballistic Method. — This method in itspresent form is due to Rowland.^ It depends upon theprinciple that when the flux of magnetic inductionthrough a coil S (Fig. 139) of Ux turns is changedby a quantity iV, the time integral of the electromotiveforce generated in the coil is n^N. If the coil S be ina circuit of resistance r, including a ballistic galva-nometer G- of long period, the quantity of electricity passing through the galvanometer will bethese quantities are in units. ri^N All of Phil. Mag., Vol. XLVI., 1873,
Electrical measurementsA laboratory manual . ulas. Fig. 138. 145. The Ballistic Method. — This method in itspresent form is due to Rowland.^ It depends upon theprinciple that when the flux of magnetic inductionthrough a coil S (Fig. 139) of Ux turns is changedby a quantity iV, the time integral of the electromotiveforce generated in the coil is n^N. If the coil S be ina circuit of resistance r, including a ballistic galva-nometer G- of long period, the quantity of electricity passing through the galvanometer will bethese quantities are in units. ri^N All of Phil. Mag., Vol. XLVI., 1873, p. 151. MAGNETISM. 309 If the same circiiit includes, as part of r, an eartli-inclnctor EI of totalarea A lying horizon-tally in a place wherethe vertical componentS^ of the earths mag-netic field is known,the constant h of thegalvanometer may bedetermined by a simplereversal of the cZi be the deflection (corrected to . 1 sm - 2 angle) correspondingto the quantity ofelectricity Q passingthrough the galvanometer; then by Art. 137. Fig. 139. Q = djc 2^9 , and k = div Let d2 be the corrected deflection due to the change iV^in the flux through S; then t^s^follows that n,N , from which it N=: 2AVd2 Uidi (1) There are several objections to -the use of the earth-inductor. In the first place, an error may be made indetermining A ; next, an error may be introduced by achange in S^, due to any one of many causes; and,thirdly, a considerable error may be introduced because 310 ELECTBICAL MEASUREMENTS. of the large number of observations necessary in deter-mining S^. For these reasons it is better to determinek by means of a standard cell and a standard JE be the electromotive force of the standard cell,and C the capacity of the condenser. Connect theapparatus as in Fig. 88, p. 188, and charge the condenserand discharge it through the galvanometer. Let d^ bethe deflection; then dji: — CE., and N=10Q ^2<^-^^^ .^^ d^sUi where C is in microfarads, E in volts, an
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