Multiplier, 1.7056; 2,110 × 1.7056 = 3,599 megohms insulation resistance per mile at 60° F. This result is recorded on the form.
VI. Copper resistance.—1. The drop of potential method is quicker than the bridge method under the usual conditions and should be used if the apparatus is available.
Apparatus required.—(a) Source of power (110 volts D. C. lighting circuit, casemate battery or generator); (b) a double-pole single-throw switch to which the power leads are attached; (c) a bank of ten 110-volt lamps in parallel; (d) a D. C. ammeter of not more than 0-25 scale; (e) a D. C. voltmeter, 0-150 scale.
Place the lamp bank and the ammeter in one side of the power line from the switch to the conductor, and the other end of the conductor to the other side of the power line. Connect the voltmeter across the ends of the cable so as to measure the drop of potential between the ends of the conductor being tested. Close the switch, take simultaneous readings on the voltmeter and the ammeter and calculate the resistance. With the apparatus described a conductor 1 mile long will receive about 2½ amperes and show a drop of about 50 volts. The lamps are inserted as a safety precaution. In no case should the current through the conductor exceed 6 amperes. If the cable has been tested for insulation resistance and all the conductors show high insulation, the lamps are not necessary, provided the cable is at least a mile long.
2. The copper resistance found is reduced to that at 60° F. by multiplying by the coefficient found in the following table with the temperature of the water in the tank at the time of the test as an argument:
| Reduction of copper resistance to 60° F. | |||
|---|---|---|---|
| Temperature. | δ | Temperature. | δ |
| °F. | °F. | ||
| 10 | 1.1252 | 55 | 1.0113 |
| 11 | 1.1224 | 56 | 1.0090 |
| 12 | 1.1196 | 57 | 1.0068 |
| 13 | 1.1168 | 58 | 1.0045 |
| 14 | 1.1141 | 59 | 1.0023 |
| 15 | 1.1113 | 60 | 1.0000 |
| 16 | 1.1086 | 61 | .9978 |
| 17 | 1.1059 | 62 | .9956 |
| 18 | 1.1032 | 63 | .9933 |
| 19 | 1.1005 | 64 | .9911 |
| 20 | 1.0978 | 65 | .9889 |
| 21 | 1.0952 | 66 | .9867 |
| 22 | 1.0925 | 67 | .9846 |
| 23 | 1.0899 | 68 | .9824 |
| 24 | 1.0873 | 69 | .9802 |
| 25 | 1.0846 | 70 | .9781 |
| 26 | 1.0820 | 71 | .9759 |
| 27 | 1.0794 | 72 | .9738 |
| 28 | 1.0769 | 73 | .9717 |
| 29 | 1.0743 | 74 | .9695 |
| 30 | 1.0717 | 75 | .9674 |
| 31 | 1.0692 | 76 | .9653 |
| 32 | 1.0667 | 77 | .9632 |
| 33 | 1.0641 | 78 | .9611 |
| 34 | 1.0616 | 79 | .9591 |
| 35 | 1.0591 | 80 | .9570 |
| 36 | 1.0566 | 81 | .9549 |
| 37 | 1.0542 | 82 | .9529 |
| 38 | 1.0517 | 83 | .9508 |
| 39 | 1.0492 | 84 | .9488 |
| 40 | 1.0468 | 85 | .9468 |
| 41 | 1.0443 | 86 | .9448 |
| 42 | 1.0419 | 87 | .9428 |
| 43 | 1.0395 | 88 | .9408 |
| 44 | 1.0371 | 89 | .9388 |
| 45 | 1.0347 | 90 | .9368 |
| 46 | 1.0323 | 91 | .9348 |
| 47 | 1.0300 | 92 | .9328 |
| 48 | 1.0276 | 93 | .9308 |
| 49 | 1.0252 | 94 | .9288 |
| 50 | 1.0229 | 95 | .9269 |
| 51 | 1.0206 | 96 | .9250 |
| 52 | 1.0182 | 97 | .9231 |
| 53 | 1.0159 | 98 | .9211 |
| 54 | 1.0136 | 99 | .9192 |
The true length of a cable should be that of its center conductor.
From the size of the conductor and its copper resistance the length of the cable may be computed by use of the following wire table:
| Table of resistances of pure copper wire at 60° F. | ||
|---|---|---|
| Size B. & S. | Dia. in mils. | Ohms per 1,000 feet. |
| 1 | 289 | 0.11999 |
| 2 | 258 | .15130 |
| 3 | 229 | .19080 |
| 4 | 204 | .24058 |
| 5 | 182 | .30338 |
| 6 | 162 | .38256 |
| 7 | 144 | .48245 |
| 8 | 128 | .60831 |
| 9 | 114 | .76696 |
| 10 | 102 | .96740 |
| 11 | 91 | 1.21960 |
| 12 | 81 | 1.5379 |
| 13 | 72 | 1.9393 |
| 14 | 64 | 2.4453 |
| 15 | 57 | 3.0134 |
| 16 | 51 | 3.8880 |
| 17 | 45 | 4.9030 |
| 18 | 40 | 6.1827 |
| 19 | 36 | 7.8024 |
| 20 | 32 | 9.8316 |
| 21 | 28.5 | 12.397 |
| 22 | 25.3 | 15.625 |
| 23 | 22.6 | 19.712 |
| 24 | 20.1 | 24.857 |
| 25 | 17.9 | 31.343 |
| 26 | 15.9 | 39.535 |
| 27 | 14.2 | 49.839 |
| 28 | 12.6 | 62.848 |
| 29 | 11.3 | 79.250 |
| 30 | 10.0 | 99.932 |
The objections to the use of a bridge for measuring copper resistance are the difficulty of eliminating the resistance of the plug contacts and the time required to secure balance. The resistance of the plug contacts may often be as high as 20 ohms, particularly if used at the tank.