Page:Popular Science Monthly Volume 63.djvu/450

446 which depend upon the action of an electrical oscillation as a magnetizing or demagnetizing agency. The third class comprises the electrolytic responders in which the action of electric oscillations either promotes or destroys the results of electrolysis. The fourth class consists of the electrothermal detectors, in which the power of an electrical oscillation as a high frequency electric current to heat a conductor is utilized. The fifth class comprises the electromagnetic or electrodynamic instruments, which are virtually very sensitive alternating current ammeters, adapted for immensely high frequency. The sixth class must be made to contain all those which cannot be well fitted at present into any of the others, such as the sensitive responder of Schäfer, the action of which is not very clearly made out.

We may proceed briefly to describe the construction of the principal forms of kumascope coming under the above headings. In the first place, let us consider those which are commonly called the 'coherers' or, as the writer prefers to call them, the contact kumascopes. The simplest of these is the crossed needle or single contact, which originated with Professor E. Branly. The pressure of the point of a steel needle against an aluminium plate was subsequently found by Sir Oliver Lodge to be a very sensitive arrangement when so adjusted that a single cell sends little or no current through the contact. When an electric wave passes over it, good conducting contact ensues. The point is, in fact, welded to the plate, and can only be detached by giving the plate or needle a light shock or vibration. A variation of the above form is a pair of crossed needles, one resting on the other.

Professor Branly found, in 1891, that if a pair of slightly oxidized copper wires rest across one another the contact resistance may fall from 8,000 to 7 ohms by the impact of an electric wave. He has recently devised a tripod arrangement, in which a light metal stool with three slightly oxidized legs stands on a polished plate of steel. The contact points must be oxidized, but not too heavily, and the stool makes a bad electrical contact until a wave falls upon it. The decoherence is effected by giving the stool a tilt by means of an electromagnet.

These single or multiple point kumascopes labor under the disadvantage that only a very small current can be passed through the variable contact when used as a relay arrangement, without welding them together so much that a considerable mechanical shock is required to break the contact and reset the trap.