(The current regulating the speed of rotation is varied by an interposed rheostat. The counter is at the top.)
into waves; how these waves carry energy from the source to a distant place; how a suitable receiver, a bell for example, is made to respond at the distant end. I now produce a quicker rotation by sending a stronger current through the motor; the frequency or pitch is raised, and the waves formed are seen to become shorter. The different frequencies are determined by means of the attached counter.
Here is a second model, a spiral spring, attached to which is a thin string. As the string is pulled, the spring is strained more and more, till the thread breaks. The spring, suddenly released, is seen to oscillate up and down. Electric vibration is produced in a somewhat similar way. If two metallic spheres be strongly charged with opposite electrifications, the medium is electrically strained, and when this strain is suddenly removed by a discharge, waves are produced in the medium.
The discharge is oscillatory, consisting of backward and for-
