446 POPULAR SCIENCE MONTHLY. 



which depend upon the action of an electrical oscillation as a magnetiz- 

 ing or demagnetizing agency. The third class comprises the electro- 

 lytic resjponderSj in which the action of electric oscillations either pro- 

 motes 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 electro- 

 dynamic 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 

 Schafer, 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 Jcumascopes. The 

 simplest of these is the crossed needle or single contact, which origi- 

 nated 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, f 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. f The 

 decoherence is effected by giving the stool a tilt by means of an 

 electromagnet. 



These single or multiple point kumascopes labor under the disad- 

 vantage 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. 



* See The Electrician, Vol. XXVII., 1891, p. 222. E. Branly, 'Variations 

 of Conductivity under Electrical Influence.' 



t See The Electrician, Vol. XL., p. 90. Sir Oliver Lodge, ' The HLstory of 

 the Coherer Principle.' 



X See Professor E. Branly, ' A Sensitive Coherer,' Comptes Rendus, Vol. 

 CXXXIV., p. 1187, 1902; or Science Abstracts, Vol. V., p. 852, 1902. 



