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.
Generally speaking, alternating current is no use for energising a magnet. The chief reason for that is that the current tends to get choked up, as it were, in the coil. Alternating current traverses a coil very reluctantly indeed. It is, however, possible to make an electric magnet of special design which will work sufficiently well with alternating current to answer our present purpose. And it will be clear that just as the alternating current itself consists of a series of short currents, so the force of the magnet will be intermittent; it will give not a steady pull, as is usually the case with magnets, but a succession of little tugs. There will, in fact, be one tug for every alternation of the current.
A simple form of motor fitted up as just described, and rotating at 3000 revolutions per minute, would give out 100 alternations per second. If, then, such current were employed to energise a magnet, that magnet would give 100 tugs per second.
So a small steel bar of the right length to give 100 vibrations per second can be fixed with its free end nearly touching such a magnet, and when the current is turned on it will very soon be vibrating vigorously. For the tugs of the magnet will agree with the natural rate of vibration of the bar. And just as the two pendulums described in Chapter XII. responded readily to each other, so the bar responds readily to the pulls of the magnet. But increase or decrease the rate of alternation ever so slightly, and that sympathy between magnet and bar is destroyed. The bar will not then respond. It will only answer when the pulls of the magnet and the natural rate of vibration of the bar exactly correspond.
So it is usual to place five or six such bars with their ends near the one magnet. The lengths of the bars vary slightly, so that the rates of vibration are, say, 98, 99, 100, 101, 102 respectively.
Let us, in imagination, adjust the speed of a supposititious motor until we get that which corresponds to 100 alternations.
We switch on the current and at first, possibly, we get no response from any of the vibrating bars. Just a touch to the handle of the rheostat and we notice that bar 102 shows signs of life. We see then that our first speed was much too fast, and that reducing it has brought it down to 102, which is still a little too fast. Just a little more movement of the handle, and 102 begins to relapse into quiet, while 101 shows animation. A little more movement and 101 gives place to 100, and then we know that our motor is working at the desired speed. If our motor had been too slow to commence with, it would have been 98 which first got into action, but the method of adjustment would have been precisely the same.
And thus we see the whole scheme. We regulate the speed by the rheostat, and meanwhile that tell-tale stream of alternating current comes flowing out of the motor to indicate to us what the speed is, while the "frequency meter," with its various vibrating bars, interprets to us the message which the alternating current brings to us. So by watching the meter we know when we have got the speed that we desire.
But even that is only half the battle. We have seen how to make a machine turn at any desired speed, and so we can adjust any two, so that they revolve at the same speed, but we have not seen how to start and stop the two machines at the same time.