Page:Electricity (1912) Kapp.djvu/184

180 both sides. By the right-hand rule stated on p. 171 we find that in the wires C1 the direction of the e.m.f. will be downwards, or away from the observer, whilst in the wires C2 it will be upwards, or towards the observer. Both these actions combined produce a potential difference between the rings R1, R2, with the result that at that moment the strength of the current flowing through the lamp will be a maximum. As the rotation proceeds, the e.m.f., and with it the current, will diminish until, when the vertical position of the coil has once more been reached, the e.m.f. will again be zero. Now the wires C1 occupy the position which formerly was occupied by the wires C2, but owing to this reversal of position the current which now starts, flows in the opposite direction. The lamp thus receives alternately current in one and the opposite direction; it is lighted with an alternating current. We have here a simple form of dynamo machine producing alternating current. The rate at which the direction of the current alternates is technically termed the "frequency." In a machine having two poles it coincides with the numbers of complete revolutions performed in one second.