Page:Proceedings of the Royal Society of London Vol 60.djvu/205

Rh The partial demagnetisation of fine steel wires, over which is wound a small solenoid, was found to be a very sensitive means of detecting electrical waves at long distances from the vibrator. Quite a marked effect was found at a distance of over half a mile from the vibrator.

Detectors made of very fine steel wire may be used to investigate waves along wires and free vibrating circuits of short wave-length. Fine wire detectors are of the same order of sensitiveness as the bolometer for showing electrical oscillations in a conductor.

This detector also has the property of distinguishing between the first and second half oscillations of a discharge, and may be used for determining the damping of electrical vibrations and the resistances of the discharge circuit.

A method of experimentally determining the period of oscillation of a Leyden jar circuit by the division of rapidly alternating currents in a multiple circuit is explained. The capacity and the self-inductance of the circuit for high frequency discharges may also be deduced, so that all the constants of a discharge circuit may be experimentally determined. In tbe course of the paper the following subjects were investigated.

(1) Magnetisation of Iron by High Frequency Discharges.—The effect of the Leyden jar discharge on soft iron and steel is fully examined. Steel needles which had been placed in a solenoid and subjected to a discharge were examined by dissolving them in acid. It was found that there was apparently only evidence of two half oscillations in the discharge, and this effect is due to the demagnetising force exerted by the needle on itself during the discharge.

The effect of continued discharges on the demagnetisation of magnetised steel needles was investigated, and also the effect of varying the length and diameter of the steel needles.

When a discharge is sent longitudinally through a magnetised steel wire the magnetic moment of the needle is always decreased, due to the circular magnetisation of the wire by the current through it. This “ longitudinal ” detector, when of thin steel wire, was found to be a sensitive means of detecting electrical oscillations of small amplitude.

Both the “ longitudinal” and “ solenoidal ” detectors may be readily used for comparing the intensities1 of currents in multiple circuits when traversed by currents of the same period.

(2) Detection of Electrical Waves at Long Distances from the Vibrator. A compound detector needle was composed of fine steel wires and a small solenoid wound over it. When this detector was placed in series with the wires of a receiver, the electrical oscillations set up in the circuit tended to demagnetise the magnetised detector needle.

By this method electrical waves from a Hertzian vibrator were