The common battery tester does not always give the exact number of amperes of current, but it indicates the relative strength which is really the thing desired. When the current from an active cell is once shown on the dial of the tester, any other cell of the same intensity will be indicated in like amount.
Electric Conductors.
—Covered wire for carrying electricity is made in a great variety of forms and designated by names that have been suggested by their use. These wires are made of a single strand or in cables, where several wires are collected, insulated and formed into a single piece. Cables may contain any number of insulated wires.
The sizes of wires are determined by a wire gage. In the United States the B. & S. gage is used as the standard for all wires and sheet metal. The gage originated with the Brown & Sharp Mfg. Co. of Providence, R. I., and has become a national standard by common consent. The numbers range from No. 0000 to No. 60. The size of wire for household electrical service ranges from No. 18 which is 0.04 inch in diameter to No. 8 which is 0.128 inch across. The carrying capacities in amperes of wires, as given by the Underwriters’ table of sizes from No. 8 to No. 18, are as follows:
| Wire gage No. | Rubber insulation, amperes | Other insulation, amperes |
| 8 | 35 | 50 |
| 10 | 25 | 30 |
| 12 | 20 | 25 |
| 14 | 15 | 20 |
| 16 | 6 | 10 |
| 18 | 3 | 5 |
Lamp Cord.
—The flexible cord used for drop lights, connectors, portable lamps, extensions, etc., is made of two cords twisted together or two cords laid parallel and covered with braided silk or cotton. The conductors consist of a number of No. 30 B. & S. gage, unannealed copper wires twisted into a cable of required capacity. The conductor is wound with fine cotton thread over which is a layer of seamless rubber, and the whole is covered with braided cotton or silk. Lamp cord is sold in three grades, old code, new code, and commercial, which vary only in the thickness and quality of rubber which encloses the conductor.
The new code lamp cord is identical with the old code form except that it is required by the National Board of Fire Underwriters to be covered with a higher quality of rubber insulation than was used in the old form. The commercial cord is not recognized by the National Board of Underwriters. It is practically the same as that described but does not conform to the tests prescribed for the new code cord.
The sizes of the conductors enclosed in the lamp cord are made equal in carrying capacity to the standard wire gage numbers. The sizes ordinarily used are No. 18 and 20 gage but they are made in sizes from No. 10 to No. 22 of the Brown & Sharp gage.