Fig. 93
We examined an ordinary 110-volt 16-candle-power carbon filament lamp, ([Fig. 93]). As near as we could estimate, its filament measured about eight inches in length. We broke open the bulb of this lamp by laying it upon the table and tapping it with a board. The bulb broke with rather a loud noise and the brittle carbon filament broke into many pieces. We found one of these pieces and measured its diameter with a wire gauge, ([Fig. 94]). It was the same size as No. 33 wire, which we also found by the wire gauge was the size of No. 90 sewing cotton. The diameter of No. 33 wire was given upon the wire gauge as .007 inch. When lighted, the filament of this lamp had looked to be about the size of No. 18 wire, which has a diameter of .04. That is, the filament when lighted looked six times as thick as it really was. Those who use sewing cotton learn quickly to know the size of the thread by its number. So those who have much to do with wire easily learn the system of designating sizes by numbers. Here are some selected figures easy to remember. A trolley wire is about one third of an inch in diameter. It is designated as No. 0. Notice in the following table that as the numbers rise by six the diameters are divided by two. Notice also that as the diameters diminish by two the resistance increases by four.
Fig. 94
| TABLE OF RESISTANCE OF COPPER WIRES | |||||||
| Nos. | Diameter | Resistance | |||||
| 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.