The number indicated by the vernier _Z_ should be read off carefully, and notice taken whether it is on the _A_ or _B_ side of the large scale. This number must then be referred to the galvanometer table,[K] when the figure opposite to the number, multiplied by the resistance unplugged, is the resistance of _X_. The value of the resistance to be determined will be thus found by a single operation. Supposing the reading to be 50 on the _A_ side of the large scale, the resistance _n_ unplugged having been 100 units, we get according to the before-mentioned law of resistance bridge the following proportion (see Fig. 5, Pl. xxiv.):-- X : 100 = 150 + 50 : 150 - 50 X = ((150 + 50) / (150 - 50)) × 100 X = 200 units. For measuring very small resistances a single cell will be found sufficient; but for large resistances more should be used, say, 15 to 20. If very accurate measurements of small resistances are to be taken, the screw at the end of the moving arm _D D_ should receive one battery wire, terminal V. receiving the other. _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. xxiv., Figs. 2 and 6. For comparing two electromotive forces _E__{1} and _E__{2}, a third electromotor of higher electromotive force _E__{0} 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 _E__{0} are to be connected to the terminals III. and V. _f._--The poles of the battery whose electromotive force _E__{1} 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-- E_{1} = E_{0} × ((150 - 30) / ( 300 + _n_)) (1), where _n_ is the resistance of the battery _E__{0}. The electromotor _E__{2} is now to be inserted in the place of _E__{1}, and the galvanometer needle, when it deflects, again brought back to zero by moving the pointer _Z_. If for instance the pointer has to be pushed to 40° on the _B_ side to obtain equilibrium we have-- E_{2} = E_{0} × ((150 + 40) / ( 300 + _n_)) (2). By eliminating _n_ from equations 1 and 2 we have E_{1} : E_{2} = (150 - 30) / (150 + 40) = 12 : 19 (3).