The difficulty, however, is to tell when the desired speed has been attained. One can count the revolutions of a machine at two or three revolutions per minute with a certain amount of accuracy, but fifty revolutions per minute are more than one could count correctly. Still less could we count the 3000 revolutions every minute of the motors. Thus, even if we had the two motors side by side, we should have extreme difficulty in making them work at the same speed exactly. One might be doing 3000 while the other did 2990 or 3010 and we should be none the wiser. And when we separate the two by a distance of many miles, the task of synchronising them is even worse. But fortunately there is a simple contrivance by which we can tell very accurately the speed of a motor. The reader has already been familiarised, in previous chapters, with the difference between direct or continuous electric currents and alternating ones. It is the continuous sort which is used to drive these motors, but a slight addition to the machine will make it so that while direct current is put in, to drive it, alternating current can be drawn out of it. Two little insulated metal rings are fitted on to the spindle of the machine, and these are connected in certain ways to the wires of the motor; then against these rings, as they turn, there rub two little metal arms, called, because of their sweeping action, brushes; and from these brushes we can draw the alternating current. For our present purpose the importance of this lies in the fact that the rate at which that current will alternate depends upon the speed of the motor. As the motor increases or decreases in speed, so will the rate of alternation increase or decrease. So that if we can measure the rate at which the current drawn from the motor is alternating, we shall know from that the rate at which the machine is working. This we can do by the aid of a "frequency meter." The working of this is based upon the acting of a tuning-fork. Everyone knows that a given tuning-fork always gives out the same note. The note depends upon the rate at which the fork vibrates, and the reason that one fork always gives the same note is because it always vibrates at the same rate. That rate, in turn, depends upon its length. If one were to file a little off the end of a tuning-fork, its note would be raised, because its rate of vibration would become faster. Similarly, lengthening the fork would result in a lower note being given. Thus, a tuning-fork, or any bar of steel held by one end, and free to vibrate at the other, gives us a standard of speed which is very reliable. And it so happens that we can easily use a set of such forks to test the rate of alternation of an alternating current.