IV. Experiments on the Dumb-bell Oscillator of
Hertz.

In the experiments previously considered an ordinary short pianoforte-wire 0.032in. in diameter acted very well as a detector, but when we come to rates of oscillation of over 100,000,000 per second a more delicate detector is required.

Some very fine steel wire was taken, glass-hard, and cut up into lengths of 1cm. Twenty-four of these little needles were then built up into, one, each being first dipped in paraffin to prevent eddy-currents passing from one wire to the other. This little collection of needles formed a compound magnet, and offered considerable surface to the action of rapidly-varying magnetizing forces. The detector was fixed in the end of a thin glass tube for convenience of handling.

This detector only retained about one-third of its magnetism, on account of the demagnetizing influence of its ends. When magnetized and placed in a solenoid of two or three turns it supplied an extremely sensitive means of detecting and measuring oscillatory currents of high frequency. It was far too sensitive to use in the ordinary leyden-jar experiments, for with one turn of wire round the tube it was completely demagnetized by a discharge.