Page:On Radiation.djvu/19

 electric current has begun to act upon them, we should find them in a state of vibration. In this vibration indeed consists such warmth as the wire then possesses. Locke enunciated this idea with great precision, and it seems placed beyond the pale of doubt by the excellent quantitative researches of Mr Joule. "Heat," says Locke, "is a very brisk agitation of the insensible parts of the object, which produce in us that sensation from which we denominate the object hot: so what in our sensation is heat in the object is nothing but motion" When the electric current, still feeble, begins to pass through the wire, its first act is to intensify the vibrations already existing, by causing the atoms to swing through wider ranges. Technically speaking, the amplitudes of the oscillations are increased. The current does this, however, without altering the period of the old vibrations, or the time in which they were accomplished. But besides intensifying the old vibrations the current generates new and more rapid ones, and when a certain definite rapidity has been attained the wire begins to glow. The colour first exhibited is red, which corresponds to the lowest rate of vibration of which the eye is able to take cognizance. By augmenting the strength of the electric current more rapid vibrations are introduced, and orange rays appear. A quicker rate