336 Professor Oliver Lodge [June 1, 



Their sensitiveness is to me surprising, though of course it does not 

 approach the sensitiveness of the eye ; at the same time, I am by no 

 means sure that the eye differs from them in kind. It is these 

 detectors that I wish specially to bring to your notice. 



Prof. Minchin, whose long and patient work in connection with 

 photo-electricity is now becoming known, and who has devised an 

 instrument more sensitive to radiation than even Boys' radiomicro- 

 meter, in that it responds to the radiation of a star while the 

 radiomicrometer does not, found some years ago that some of his 

 light-excitable cells lost their sensitiveness capriciously on tapping, 

 and later he found that they frequently regained it again while 

 Mr. Gregory's Hertz- wave experiments were going on in the same 

 room. 



These " impulsion-cells," as he terms them, are troublesome 

 things for ordinary persons to make and work with — at least I have 

 never presumed to try — but in Mr. Minchin's hands they are 

 surprisingly sensitive to electric waves.* 



The sensitiveness of selenium to light is known to every one, and 

 Mr. Shelford Bidwell has made experiments on the variations of 

 conductivity exhibited by a mixture of sulphur and carbon. 



Nearly four years ago M. Edouard Branly found that a burnished 

 coat of porphyrised copper spread on glass diminished its resistance 

 enormously, from some millions to some hundreds of ohms when it 

 was exposed to the neighbourhood, even the distant neighbourhood, 

 of Leyden jar or coil sparks. He likewise found that a tube of 

 metallic tilings behaved similarly, but that this recovered its original 

 resistance on shaking. Dr. Dawson Turner exhibited this fact recently 

 at the Edinburgh meeting of the British Association, and Mr. Croft 

 has shown it to the Physical Society. M. Branly also made pastes 

 and solid rods of filings, in Canada balsam and in sulphur, and found 

 them likewise sensitive.f 



With me the matter arose somewhat differently, as an outcome 

 of the air-gap detector employed w r ith an electroscope by Boltzmann. 

 For I had observed in 1889 that two knobs sufficiently close together, 

 far too close to stand any voltage such as an electroscope can show, 

 could, when a spark passed between them, actually cohere ; conducting 

 an ordinary bell-ringing current if a single voltaic cell was in circuit ; 

 and, if there w 7 ere no such cell, exhibiting an electromotive force 

 of their own sufficient to disturb a low resistance galvanometer 

 vigorously, and sometimes requiring a faintly perceptible amount of 

 force to detach them. The experiment was described to the Institution 

 of Electrical Engineers, J and Prof. Hughes said he had observed the 

 same thing. 



Well, this arrangement, wiiich I call a coherer, is the most 



* Phil. Mag., vol. xxxi. p. 223. 



t E. Branly, 'Comptes Rendus,' vol. cxi. p. 785 ; and vol. cxii. p. 90. 

 \ 'Journal' Institution of Electrical Engineers, 1890, vol. xix. pp. 352-4; or 

 'Lightning Conductors and Lightning Guards ' (Whittaker), pp. 382-4. 



