270. The Voltmeter.—An instrument for measuring the electromotive force of electric currents is called a voltmeter (Fig. 249). It is usually a moving-coil galvanometer, and is always of high resistance. It is like an ammeter in construction and appearance. In fact, a voltmeter is an ammeter which has had its shunt removed or disconnected. In place of a shunt, the voltmeter uses a coil of wire of high resistance (see R, Fig. 249) in series with the galvanometer coil. The high resistance of the voltmeter permits but a very small current to flow through it. Hence a voltmeter must be placed across a circuit and not in it. In other words a voltmeter is connected in shunt, while an ammeter is in series with the circuit as is shown in Fig. 250.
Dimensions and Functions of Copper Wires
| B. & S. gauge number | Diameter | Circular mils | Sectional area in square millimeters | Weight and length, Density = 8.9, feet per pound | Resistance at 24°C., feet per ohm | Capacity in amperes | |
| Mils | Millimeters | ||||||
| 0000 | 460.000 | 11.684 | 211,600.00 | 107.219 | 1.56 | 19,929.700 | 312.0 |
| 000 | 409.640 | 10.405 | 167,805.00 | 85.028 | 1.97 | 15,804.900 | 262.0 |
| 00 | 364.800 | 9.266 | 133,079.40 | 67.431 | 2.49 | 12,534.200 | 220.0 |
| 0 | 324.950 | 8.254 | 105,592.50 | 53.470 | 3.13 | 9,945.300 | 185.0 |
| 2 | 257.630 | 6.544 | 66,373.00 | 33.631 | 4.99 | 6,251.400 | 131.0 |
| 4 | 204.310 | 5.189 | 41,742.00 | 21.151 | 7.93 | 3,931.600 | 92.3 |
| 6 | 162.020 | 4.115 | 26,250.50 | 13.301 | 12.61 | 2,472.400 | 65.2 |
| 8 | 128.490 | 3.264 | 16,509.00 | 8.366 | 20.05 | 1,555.000 | 46.1 |
| 10 | 101.890 | 2.588 | 10,381.00 | 5.260 | 31.38 | 977.800 | 32.5 |
| 12 | 80.808 | 2.053 | 6,529.90 | 3.309 | 50.69 | 615.020 | 23.0 |
| 14 | 64.084 | 1.628 | 4,106.80 | 2.081 | 80.59 | 386.800 | 16.2 |
| 16 | 50.820 | 1.291 | 2,582.90 | 1.309 | 128.14 | 243.250 | 11.5 |
| 18 | 40.303 | 1.024 | 1,624.30 | 0.823 | 203.76 | 152.990 | 8.1 |
| 20 | 31.961 | 0.812 | 1,021.50 | 0.5176 | 324.00 | 96.210 | 5.7 |
| 22 | 25.347 | 0.644 | 642.70 | 0.3255 | 515.15 | 60.510 | 4.0 |
| 24 | 20.100 | 0.511 | 504.01 | 0.2047 | 819.21 | 38.050 | 2.8 |
| 26 | 15.940 | 0.405 | 254.01 | 0.1288 | 1,302.61 | 23.930 | 2.0 |
| 28 | 12.641 | 0.321 | 159.79 | 0.08097 | 2,071.22 | 15.050 | 1.4 |
| 30 | 10.025 | 0.255 | 100.50 | 0.05092 | 3,293.97 | 9.466 | 1.0 |
| 32 | 7.950 | 0.202 | 63.20 | 0.03203 | 5,236.66 | 5.952 | 0.70 |
| 34 | 6.304 | 0.160 | 39.74 | 0.02014 | 8,328.30 | 3.743 | 0.50 |
| 36 | 5.000 | 0.127 | 25.00 | 0.01267 | 13,238.83 | 2.355 | 0.35 |
| 38 | 3.965 | 0.101 | 15.72 | 0.00797 | 20,854.65 | 1.481 | 0.25 |
| 40 | 3.144 | 0.080 | 9.89 | 0.00501 | 33,175.94 | 0.931 | 0.17 |
Important Topics
(1) Galvanometers: (1) moving magnet, fixed coil; (2) moving coil, fixed magnet, ammeter, voltmeter.
(2) Unit of quantity, coulomb.
(3) Unit of current, ampere.
(4) Unit of resistance, ohm.
(5) Unit of electromotive force, volt.