234 Prof. Minchin's Experiments in Photoelectricity. 



the long diameter, N N, of the needle, and terminating in a 

 sphere, p ; the portion mr of the wire dips into a shallow 

 dish, d, of mercury which is supported below the quadrants 

 by an arm not represented ; a platinum wire, t, dips into the 

 dish d, and, passing outside the case of the electrometer, is 

 connected with one end of the coil of a small electromagnet, e, 

 whose armature is a ; the other end of this coil is connected 

 with one pole of a weak voltaic cell, b, whose other pole is 

 connected with a small platinum plate, s, through a sup- 

 port, h, which is fixed inside the case of the electrometer, and, 

 of course, insulated from the needle and the quadrants ; the 

 platinum plate s is fixed tolerably close to the position of rest 

 of the platinum point p ; and when the needle is deflected by 

 the E.M.F. of a photoelectric battery, P, connected with the 

 poles, C, D, of the electrometer, the point p comes into contact 

 with the plate s and completes the circuit of the voltaic 

 cell b, and thus brings the armature a into contact with the 

 magnet of the coil e : this connexion completes the circuit of 

 a voltaic battery, B, in whose circuit is an incandescent 

 lamp, L, or an electric bell, which is then set in action. 

 The needle is connected with the sulphuric acid jar, J, by 

 means of a fine platinum wire, w, bent into a semicircular 

 form so as to avoid contact with the dish d when the needle 

 moves, the wire w terminating in a vertical length exactly 

 under the centre of the needle ; thus, since the wires r and v 

 are in the vertical axis of rotation of the needle, no appreci- 

 able friction hinders the motion of the needle. 



The needle is suspended by two fine silk fibres,/,/, from a 

 support A inside the case of the electrometer. Instead of 

 the two external voltaic batteries, B, b, one would theoreti- 

 cally suffice ; but it is found that, to prevent sparking and 

 "stiction" between^? and s when the current is made, it is 

 preferable to have a very weak current traversing this portion 

 of the arrangement — one which is just sufficient to work the 

 electromagnet e. 



In this way, by means of a few seleno-aluminium cells, P, 

 I have found no difficulty in ringing a bell by the light of a 

 taper or that of a match held at a distance of a few feet from 

 the battery P. Observe that by this method we never draw 

 on the materials of the photo-battery, because no current 

 ever passes through it ; it is simply connected with the poles 

 of the electrometer, and its E.M.F. alone is employed. 



The Problems of Photoelectricity. 



Three prominent problems in this subject deserve to be 

 signalized. The first, and least pretentious, is the construe- 



