. A text book of physics, for the use of students of science and engineering . the box is the sum of the separate resistances of thecoils. By inserting any plug, the corresponding coil is short cir-cuited, so that when allthe plugs are in, the totalresistance is practicallyzero. By making the coilsto have respectively 1, 2,3, 4, 10, 20, 30, 40, 100,200, 300, 400, 1000, 2000,3000 and 4000 ohms, anyresistance from 0 to 11110may be used. In somecases fractions of an ohmare also supplied. In is shown a view ofsuch a resistance, are many forms ofresistance box. Comparison of elect
. A text book of physics, for the use of students of science and engineering . the box is the sum of the separate resistances of thecoils. By inserting any plug, the corresponding coil is short cir-cuited, so that when allthe plugs are in, the totalresistance is practicallyzero. By making the coilsto have respectively 1, 2,3, 4, 10, 20, 30, 40, 100,200, 300, 400, 1000, 2000,3000 and 4000 ohms, anyresistance from 0 to 11110may be used. In somecases fractions of an ohmare also supplied. In is shown a view ofsuch a resistance, are many forms ofresistance box. Comparison of electro-motive The sim-plest method of comparingthe of two cells isto find the current which each would produce in a given placing one of the cells, of Ex, in series with a resistancebox R and a galvanometer G (Fig. 817) the current is EX/(G + R), andtlir deflection observed may lie written 0V On replacing the cell Exby another cell whose is E2, the current is E2/(G + R) andthe deflection is 62. If the deflections are proportional to the. Fig. 816.—Resistance box I lxix COMPARISON OF ELECTROMOTIVE FORCES 881 i currents, E1/E2 = dlld2, or if a tangent galvanometer be employed, I E1/E2 = tan djtsna. d*. Or, again, if a simple galvanometer, which I has previously been calibrated, is used, the I currents may be taken from the calibration ^_f~? ? curve (p. 863) and then E1/E2 = I1/I2. Another method is to obtain the same deflec-I tion with each cell in turn, by varying theI resistance R. Then, since the current in eachI case is the same, EX/(G + Rj) = E2/(G + R2). WhenI Rx and R2 are great in comparison with theresistance G of the galvanometer,
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