| Silver annealed | 1.00 |
| Copper annealed from | 1.04 |
| (Copper annealed) to | 1.09 |
| Aluminum annealed | 1.64 |
| Nickel annealed | 4.69 |
| Platinum annealed | 6.09 |
| Iron annealed | 6.56 |
| German Silver from | 12.80 |
| (German Silver) to | 20.20 |
| Mercury | 63.30 |
| Nichrome | 67.50 |
| Carbon from | 2700.00 |
| (Carbon) to | 6700.00 |
267. The ohm, the unit of resistance, is defined by international agreement as follows: An ohm is the resistance of a column of pure mercury, 106.3 cm. long with a cross-section of a square millimeter and at a temperature of 0°C.
It should be noted that each of the four conditions affecting resistance is mentioned in the definition, viz., length, cross-section, material, and temperature. Since it is inconvenient to handle mercury, standard resistance coils, made of an alloy of high resistance are used in comparing and measuring resistances.
A piece of copper wire No. 22 (diameter 0.644 mm.) 60. 5 ft. long has a resistance of 1 ohm. See table p. 296.
The resistance of some telephone receivers is 75 ohms, of a telegraph sounder, 4 ohms, of a relay 200 ohms.
268. Resistance of Circuits.—Every part of an electrical circuit possesses resistance. In an electric-bell circuit, for instance, the wires, the bell, the push-button, and the cell itself, each offers a definite resistance to the passage of the current. The resistance within the cell is termed internal resistance, while the resistance of the parts outside of the electric generator is called external resistance.
269. Electromotive Force.—In order to set in motion anything, some force must be applied. This is as true of electricity as of solids, liquids, or gases. By analogy that which is exerted by a battery or by a dynamo in causing current to flow is called an electromotive force. The unit of electromotive force, the volt, may be defined as the electromotive force that will drive a current of 1 ampere through the resistance of 1 ohm. The electromotive force of a dry cell is about 1.5 volts, of a Daniell cell 1.08 volts. Most electric light circuits in buildings carry current at 110 or 220 volts pressure. Currents for street cars have an electromotive force of from 550 to 660 volts.
Fig. 249.—Diagram of a commercial voltmeter.