Study of the Cohering Action of different Metals. 167 



given metal with gradually increasing pressure, this increase of pres- 

 sure being capable of the finest adjustments. The circuit was com- 

 pleted through the metal and mercury. Sometimes the variation of 

 pressure was produced by a pressure bulb. In the arrangement 

 described above the contact is between different metals and mercury — 

 metals which were even amalgamated by mercury still exhibited 

 sensitiveness to electric radiation when the amalgamation did not pro- 

 ceed too far. In this way I was able to detect the cohering action of 

 many conductors, including carbon. For studying the contact-sensitive- 

 ness of similar metals I made an iron float on which was soldered a 

 split-tube in which the given metal could be fixed, a similar piece of 

 metal being adjusted above the float, so that by working the plunger 

 or the pressure bulb the two metals could be brought into contact with 

 graduated pressure. The other arrangements adopted were the contact 

 of spirals compressed by micrometer screw, and filings similarly com- 

 pressed between two electrodes. 



With the arrangement described above the action of radiation on 

 metallic contacts was studied, a brief account of which will be given 

 under their respective groupings. It may here be mentioned that 

 certain metals which do not usually show any contact-sensitiveness can 

 be made to exhibit it by very careful manipulation. The nature of the 

 response of a coherer is to a certain extent modified by its condition 

 and particular adjustment. A coherer freshly made is more difficult to 

 adjust, but at the same time far more sensitive. The action is more 

 easily under control and more consistent after a few days' rest, but the 

 sensitiveness is not so abnormally great. The contacts of bright and 

 clear surfaces are difficult to adjust, but such contacts are more sensi- 

 tive than those made by tarnished surfaces. Pressure and E.M.F., as 

 previously stated, also modify the reaction. For example, a freshly 

 made and very delicately adjusted coherer subjected to slight pressure 

 and small E.M.F. showed an increase of resistance by the action of 

 radiation. The galvanometer spot, after a short interval, resumed its 

 former position, exhibiting a recovery from the effect of radiation. 

 The coherer continued to exhibit this effect for some time, then it 

 relapsed into the more stable condition in which a diminution of 

 resistance is produced by the action of radiation. Another coherer 

 was found apparently irresponsive to radiation, there being the merest 

 throb (sometimes even this was wanting) in the galvanometer spot, 

 when a flash of radiation fell on the receiver. Thinking that this 

 apparent immobility of the galvanometer spot may be due to response, 

 followed by instantaneous recovery, the galvanometer needle being 

 subjected to opposite impulses in rapid succession, I interposed a tele- 

 phone in the circuit ; each time a flash of radiation fell on the receiver 

 the telephone sounded, no tapping being necessary to restore the sensi- 

 tiveness. The recovery was here automatic and rapid. After twenty 



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