0 .32 inch 10560 feet to the ohm 6 .16 " 2640 " " " " 12 .08 " 660 " " " " 18 .04 " 165 " " " " 24 .02 " 40 " " " " 30 .01 " 10 " " " " 36 .005 " 2.5 " " " " 42 .003 " 1 " " " " 10,560 feet equal two miles. Number 36 is the wire used upon the spools of telegraph receivers. They offer 75 ohms of resistance and therefore contain 30 feet of wire (30 × 2.5 = 75). These resistances are for ordinary school room temperatures. 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. [Illustration: Fig. 95]