Comparing Electromotive Forces.—For this purpose Professor E. du Bois-Reymond's modification of Poggendorff's compensation method is used.
The connections are made as shown at Pl. xxiiiA., [Fig. 2] and [6].
For comparing two electromotive forces E1 and E2, a third electromotor of higher electromotive force E0 is used, and two separate tests taken.
The manipulations a and b are to be the same as before.
c.—The hole between III. and IV. to be left unplugged.
d.—Plugs to be inserted in 10, 100 and 1000.
e.—The two poles of the electromotor of an electromotive force E0 are to be connected to the terminals III. and V.
f.—The poles of the battery whose electromotive force E1 is to be compared are connected to terminals I. and IV. in such a manner that the similar poles of the two electromotors are joined to terminals I. and III., and to IV. and V. respectively.
When depressing the key K the galvanometer needle will be deflected and can be brought back to zero by turning the pointer Z either to the right or to the left. Should for instance the pointer have to be brought to 30° on the A side we have the following equation—
| E1 = E 0 | 150 - 30 | . . . . . . . . (1), |
| 300 + n |