[ 63 ] 



X. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



[Continued from vol. xxxii. p. 544.] 



May 31, 1866.— Dr. W. A. Miller, Treas. & V.P., in the Chair. 



HjlHE following communication was read: — 



1 "On the means of increasing the Quantity of Electricity 

 given by Induction-Machines." By the Rev. T. Romney Robinson, 

 D.D. 



Among the remarkable results obtained by studying the spectra of 

 electric discharges, is the change exhibited by certain substances when 

 the nature of the discharge is varied. In general the mere spark 

 shows fewer and fainter lines than when a Ley den jar is in connexion, 

 though the amount of electricity supplied by the machine is the 

 same. In the latter case, however, the discharge passes almost 

 instantaneously, and therefore its concentrated action will be more 

 powerful. But, as far as I know, much has not been attempted to- 

 wards increasing the power of the jar : this cannot be done by in- 

 creasing its surface (unless indeed that be too small to condense all 

 the electricity supplied) ; the supply itself must be increased. 



This may be done in three ways : — 



First, the power of the exciting battery may be increased. This, 

 however, is limited by the risk of destroying the acting surfaces of 

 the rheotome ; and by the decreasing rate at which the magnetism 

 of the iron core increases with the primary current. In some in- 

 vestigations on the electromagnet (Trans. Irish Academy, vol. xxiii. 

 p. 529) I have shown that its lifting power L is approximately given 

 by the equation 



T _ A¥ 



^-bT*' 



in which •*" is the product of the current and number of spires, 

 A the maximum lift of the magnet, and B the "ir which would excite 



it to half A. The rate of change — is therefore inversely as 



(B + ^) 2 . The results obtained with two of the magnets which I used 

 will illustrate this. Their A's are 781 lbs. and 278 lbs. The first 

 1000 of ^ make their lifts 576 and 235 ; the second 1000 adds to 

 these 87 and 19 ; the third 35 and 8 ; and the fourth only 19 and 

 3. With a primary of 180 spires, ^=4000 implies a current which 

 can evolve in a voltameter 34*7 cubic inches of gases per minute, and 

 of course has great deflagrating power. There is therefore not much 

 to be gained in this direction. 



Secondly, the secondary helix may be made of longer or thicker 

 wire. It will be shown immediately that the length does not increase 

 the quantity of the current at all, and that the effect of the thickness 

 is limited. 



