224 Prof. Minchin's Experiments in Photoelectricity. 



I am not able to produce the reverse effect — the change 

 from the sensitive to the insensitive state — by electromag- 

 netic induction ; dull taps administered to the base of the 

 cell constitute the only way in which this change can be 

 produced with certainty. On very rare occasions the change 

 has been effected by the inductive action of strong sparks 

 from a Leyden jar ; but the result is quite exceptional. 

 There appears to be some reason for supposing that this 

 change — from sensitive to insensitive — is produced by vibra- 

 tions of slow period, or very dull taps. I have produced it 

 by dropping very small pieces of cork on the base of the iron 

 retort-stand in which the cell is held, and even by gently 

 drawing a piece of paper across the retort-stand. 



Nearly all these tin-foil cells will develop the impulsion 

 character a few days, or weeks, after they have been formed, 

 provided that the alcohol in them has not been thrown out and 

 replaced by fresh alcohol. Three years ago I formed a battery 

 of 30 of these little cells, testing each before adopting it in 

 the battery, and connected them in series, hoping thereby to 

 obtain a very large E.M.F. on exposure to light. To my 

 disappointment, however, I found a comparatively small 

 result ; and on examining the cells individually, I found 

 some of them insensitive. Their sensitiveness was restored 

 by renewing the alcohol, but it might have been restored by 

 impulses if I had known the fact. 



One of this battery in which the impulsion results appeared 

 was taken out and its alcohol renewed on 4 or 5 days suc- 

 cessively, with a view to ascertaining whether the impulsion 

 results could be got rid of by renewing the liquid. After 

 six renewals the cell ceased to give impulsion-effects, and it 

 has been since under trial from time to time for about three 

 years. 



Impulsion-cells are very strongly influenced if they are 

 kept in a room in which powerful sparks are being produced 

 from a Holtz machine. A box of them which had been put 

 by, for exhibition before this Society, in a cupboard about 

 six feet distant from a Holtz which was put in action, was 

 found during the process to have had nearly every cell affected 

 in such a manner that no impulsion-effects could be produced 

 for several hours after the Holtz ceased to be worked ; and a 

 strikingly good example of an impulsion-cell which I was 

 very anxious to preserve acquired a strong tendency to revert 

 from the sensitive to the insensitive state, and to remain in 

 the latter ; at the same time it lost its extreme sensitiveness 

 to impulses. 



The experiment with a galvanometer, previously men- 



