Alternating current, its frequency and the hot-wire ammeter
Alternating current (AC) changes direction regularly, and its value follows a sine curve. The frequency f is the number of complete cycles in one second, in hertz (Hz), and the period is T = 1 ÷ f. The supply in Egypt has f = 50 Hz, so T = 1 ÷ 50 = 0.02 s, and the current changes direction twice in each cycle, that is 100 times in every second. AC is used for supply networks because transformers can raise and lower its voltage, which makes transmission over long distances efficient, because it can be changed into DC (rectified) when a device needs DC, and because its heating effect does not depend on the direction of the current. It is not suitable for electrolysis or electroplating, which need a current in one direction. A moving-coil ammeter cannot read AC: the torque on its coil reverses with the current 100 times a second, so the pointer only trembles about zero. The hot-wire ammeter works by heating. The current warms a thin wire, the wire expands, and a thread held by a spring moves the pointer. The heating is proportional to I², so the instrument reads the effective value of an AC (and the ordinary value of a DC). Its scale is crowded at the low end, it is slow, and its zero drifts with the room temperature. We describe it only in words and drawings.
| Quantity | Value |
|---|---|
| f | 50 Hz |
| T | 0.02 s = 20 ms |
| reversals per second | 100 |
Paper only. Draw one full cycle of an alternating current and mark the period T. For f = 50 Hz mark T = 20 ms and the half cycle at 10 ms. Then draw the hot-wire ammeter as a thin wire between two fixed points with a thread, a spring and a pointer, and write beside it what happens when the current is switched on.