Prof. Minchin's Experiments in Pliotoelectricity. 223 



affairs in a cell which was under experiment. It was found 

 that on exposing the upper portion, OP, ot the plate and 

 screening the lower, a negative deflexion on the scale was 

 observed — that is, a deflexion which indicated that the 

 exposed plate was negative ; while if the upper portion was 

 screened and the lower exposed, a positive deflexion resulted. 

 Then on giving a slight tap to the support of the cell and 

 exposing the portion CP, while screening the lower, a positive 

 deflexion was observed ; and on exposing the lower portion 

 alone, a positive E.M.F. resulted, as before. Thus the effect 

 of the vibration was to alter the nature of the upper portion 

 while leaving the lower unaltered. 



In another cell, which I now exhibit to the Society, the 

 sensitive plate was completely immersed, and there was, as in 

 the last case, a certain portion, OP, at the upper end which 

 was rendered alternately positive and negative by vibrations, 

 while the lower portion remained unaltered. These facts 

 seem to be quite inconsistent with a want-of-contact theory. 

 Before citing another experiment, in which a Thomson galva- 

 nometer is employed, to disprove such a theory, it is well to 

 mention a remarkable method of producing the sensitive state 

 from the insensitive. 



While investigating the effect of static charges communi- 

 cated to the plates on the sensitive and insensitive states, I 

 found that if a Voss machine, not in any way connected with 

 the cell or the electrometer, was worked in the room while 

 the cell was in the insensitive state, the moment a spark 

 passed between the poles of the Voss, the insensitive state ivas 

 altered to the sensitive, whether the cell was connected with 

 the electrometer or not. The same effect was produced at a 

 much greater distance from the cell by the inductive action 

 of the spark passing between the two poles of the secondary 

 coil of an induction-coil, as in a Hertz oscillator arrangement. 

 In fact when a Hertz oscillator was taken into the grounds 

 outside the laboratory in which I worked, the induction-coil 

 being actuated by a battery of 4 or 5 Grove cells, no wires 

 whatever passing from this machine near the laboratory, the 

 insensitive cell inside the room at a distance of 81 feet was 

 instantly rendered sensitive by the inductive action of the 

 Hertz arrangement. 



Impulsion-cells differ much from each other in the readi- 

 ness with which they undergo the change from one state to 

 the opposite ; but the most obstinate can be thrown from 

 the insensitive into the sensitive state by leading a wire 

 connected with either pole into the vicinity of a Hertz or a 

 Voss machine. 



