tJOtJ 



REPORT — 1901. 



this wir(j is surrouudecl by a tightly fittiug glass tube a very great voltage is 

 produced by light. 



The accompanying figure represents the two aluminium wireSj A and B, cou- 

 tained in a cork which fits tightly into a glass cell, C, containing a liquid ; the 

 wires are each contained in a tightly fitting glass tube and are connected with the 

 poles of a quadrant electrometer, E. The ends of the wires are exactly flush with 

 the ends of the glass tubes, which dip into the liquid, and these ends are coated 

 with the selenium layers. We shall suppose that the wire B is screened from the 

 incident light L. Each aluminium wire is abont | mm. or ^ mm. thick, nothing 

 depending on the diameter of the wire — i.e., on the area of the sensitive selenium 

 surface — provided that the w/iole of this surface is illuminated by the incident light. 



Now the question will naturally occur, Why do we use aluminium and not 

 some other metal, e.f/., platinum, as the base for the selenium ? The]answer is that 

 many other metals have been tried, and none of them gives results approaching 

 those given by aluminium. Platinum develops only about half the voltage wliich, 

 under the same circumstances, will be given by aluminium. Metals with which 

 selenium combines readily are useless : copper, silver, and tin are very poor. Now 

 as regards the liquids which are most efl'ective, I have found the following to be 

 extremely good : ceuauthol, acetone, succinate of ethyl, malouic ether, methyl- 



-W.'/;j/;,l i:,i 



WMMK. 



v/;mwm g- 



hexylketone, ethyl and methyl ben/.oate, methyl carbonate, lactic acid, lactate of 

 ethyl, and lactate of methyl. 



Cyanide of ethyl is somewhat efiective ; but such liquids as anisol, mustard oil, 

 ethyl acetate, valerate of ethyl, and valerate of methyl are not sensitive at all. 



Within the last few Aveeks, however, I discovered a fact which will certainly 

 modify some of my statements about the want of sensitiveness of liquids — the fact, 

 namely, that nearly all of the little glass tubes which surrounded the aluminium 

 wires,"and on which I had relied for insulating these wires from the liquid, were 

 very good conductors ! I found that if the electrometer was charged by a Daniell 

 cell, which was then withdrawn, so that the poles of the electrometer were 

 insulated, one of my glass tubes laid across from one pole to the other rapidly 

 discharged the electrometer ; and drying the glass tube did not much improve its 

 insulation. As a result of this, I have recently used a compound glass tube — 

 one tube inside another with a layer of air between them, except at a common 

 extremity where they are sealed together, thus : 



The inner tube is P, sealed to the Outer, QQ, 



Q at the end S, where the selenium surface of the 



S 1 .^ ~ :z. P wire contained in the inner tube comes into 



Ci contact with the liquid. 



Except in the neighboui'hood of S, this 

 interposes a layer of air between the tube P and the liquid, and the result is a 

 great improvement. 



A more efl'ective way still is to coat the aluminium wire with an insulating 

 varnish ; but this varnish must be such as not to be dissolved by the liquid which 

 we employ. 



I am now engaged on this part of the problem, and it is mainly this which has 

 compelled me to delay the star measurements which I was to have resumed at Sir 



