Since iron has six times, and German silver twelve times the resistance of copper, divide the figures of the third column by six, and the table will answer for iron wire, or divide those figures by twelve and the table may be used for German silver wire, thus:
| Number Feet to the Ohm | ||||||
| Nos. | Diameter | Copper | Iron | German Silver | ||
| 0 | .32 | inch | 10560 | 1760 | 880 | |
| 6 | .16 | " | 2640 | 440 | 220 | |
| 12 | .08 | " | 660 | 110 | 55 | |
| 18 | .04 | " | 165 | 27 | 14 | |
| 24 | .02 | " | 40 | 6 | 32 | inch |
| 30 | .01 | " | 10 | 1.5 | 8 | " |
| 36 | .005 | " | 2.5 | .45 | 2 | " |
| 42 | .003 | " | 1 | 2 inch | 1 | " |
These figures are not exact, but useful.
We procured a string of eight small lamps ([Fig. 95]), such as are used in lighting Christmas trees. Each was marked 14 volt, 2-candle-power. The carbon filament of each was about one inch long and apparently the same diameter as that of the 16-candle-power lamp. When the 110-volt current was sent through the group of eight connected in series they seemed to give about the same light as the single 16-candle-power lamp. It is as though the filament of the 16-candle-power lamp had been cut into eight pieces, and distributed through eight small lamps. We introduced an ammeter into the circuit and found that half an ampere of electricity passed through the single 16-candle-power lamp—and half an ampere likewise passed through the group of eight 2-candle-power lamps.
Fig. 95
The 110-volt current can push an ampere of electricity through eight inches of carbon thread seven thousandths of an inch in diameter, and when this happens the filament gets hot enough to give out as much light as sixteen standard candles. In the place of the 16-candle-power lamp, we put a 32-candle-power 110-volt lamp. The ammeter indicated one ampere. The carbon filament was larger (No. 30, diameter = .01 inch), so as to allow more current to pass. An 8-candle-power 110-volt lamp was substituted; one quarter of an ampere passed. A 4-candle-power 110-volt lamp was used; one eighth of an ampere passed. A 100-candle-power 110-volt lamp was substituted; three amperes of current passed through it. In all these cases the lamps which passed the larger current had the larger filaments. A little practice would enable one to distinguish between these lamps without labels by examining their filaments. Among these 110-volt lamps, it is to be noted that the amount of light which they give is proportional to the amount of current which they pass. And it is convenient to remember that one ampere of electricity for one hour costs about one cent.