c.—A plug is to be inserted between the terminals marked III. and IV.
d.—The holes 10, 100, and 1000 are, two of them, to be plugged, and one left open, according to the extent of the unknown resistance to be measured; either 10 or 100 must be left open if the resistance is small, and 1000 if it is large.
e.—The two ends of the unknown resistance are to be connected to terminals II. and IV.
f.—The two poles of some galvanic battery are to be connected to terminals I. and V.
When the above-mentioned connections have been made, and on depressing the key K, the battery current is sent into the combination and deflects the needle, say, to the right-hand or B side of the instrument, the pointer or vernier Z must then be pushed, by means of the handle g, to the B side of the instrument. If this is found to increase the deflection of the needle i, the pointer Z should be pushed to the other or A side of the instrument beyond the zero point of the large scale until the needle remains stationary when the key K is depressed.
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. xxiiiA.):—
| X : | 100 | = | 150 + 50 | : 150 - 50 |
| X | 150 + 50 | × 100 | ||
| 150 - 50 | ||||
| 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.