style or marker. These describe undulations on the cylinder, and the undulations correspond to equal periods of time. No instruments vibrate with greater uniformity than tuning-forks. Duhamel was the first to apply to one of the limbs of a tuning-fork a small marker, and to bring this marker against a rapidly moving surface, like the surface of this blackened cylinder. Undulations or waves are thus described. The more rapid the movement of the cylinder, the longer will be the waves, as you see in the experiment, and as are represented in this diagram (Fig. 26).
It is difficult to apply the vibrating limb of a tuning-fork to a cylinder, more especially if other recording apparatus is adjusted to the cylinder at the same time. Suppose we use the fork simply for interrupting the current, and this it will do with great regularity, we might interpolate in the circuit a little electromagnetic appliance, having a keeper, to which a marker is attached. This is the apparatus you see here. It consists of a battery, an interrupting tuning-fork, and an electromagnetic instrument called the chronograph or marker. The chronograph consists of a fine
