with a glass front. Through the glass one sees a row of little white knobs. If this be placed somewhere near the engine while it is at work immediately one of the knobs commences to move rapidly up and down, so that it looks no longer like a knob but is elongated into a white band. There is no visible connection between the instrument and the engine, yet the number over that particular knob which becomes thus agitated indicates the speed of the engine.
Let us in imagination open the case and we shall find that the knobs are attached to the ends of a number of light steel springs set in a row. The springs are all precisely alike except for their length, in which respect no two are alike. Indeed, as you proceed from one side of the instrument to the other each succeeding one is a little longer than the previous one. Now a spring has a certain speed at which it naturally vibrates and other things being equal that speed depends upon its length. You can, of course, force any spring to vibrate at any speed if you care to take the trouble, but each one has its own natural speed at which it will vibrate under very slight provocation.
Every engine is, of course, made to run as smoothly as possible. All revolving or reciprocating parts are for this reason carefully balanced and in turbines the whole moving part, since it is round and symmetrical, naturally approaches a condition of perfect balance. Hence every engine ought to run perfectly smoothly. As a matter of fact, however, no engine ever does. There are certain limitations to man's skill and at the high speed of a fast-running engine, such as is to be
found on a destroyer, for example, some little irregularity is sure to make itself felt by a slight vibration in the floor. It may be hardly perceptible to the senses, but to a spring whose natural frequency happens to be just that same speed or nearly so, it will be very apparent and in a few seconds that spring will be responding quite vigorously.
It is another example of the principle of resonance, which is employed so finely in making wireless telegraph apparatus selective. Every wireless apparatus is made to have a certain natural frequency of its own and it therefore picks up readily those signals which proceed from another station having the same frequency while ignoring those from others. In just the same way a reed or spring in this speed-indicator picks up and responds to impulses derived from the engine only when they are of a frequency corresponding with its own natural frequency. Hence, one spring out of the whole range responds to the vibrations of the engine while the others remain almost if not entirely unaffected.
In another form, the springs are actuated electrically. A magnet, or a series of magnets, is arranged so that as the engine turns the magnets pass successively near to a coil of wire, thereby inducing currents in that wire. They form, in fact, a small dynamo or generator, generating one impulse per revolution or two or three or whatever number may be most convenient. Then the current from this is led round the coil of a long electro-magnet placed just under the free ends of all the springs. The magnet therefore gives a series of pulls, at regular
intervals, and the rapidity of those pulls will depend upon the speed of the engine, while the frequency of them will be registered by the movement of one or other of the springs.
This instrument can also be employed to determine the speed of aeroplane motors and, in fact, any kind of engine, especially those whose speed is very high.