1866.] On increasing the Electricitij of Induction-Machines . 171 



where y is the ratio of the two specific heats, the pressure, and the 

 density of the standard gas at absolute temperature Oq. S the specific 

 gravity of the gas in question, and [i. its viscosity. The conductivity is, 

 hke the viscosity, independent of the pressure and proportional to the ab- 

 solute temperature. Its value for air is about 3500 times less than that 

 of wrought iron, as determined by Principal Forbes. Specific gravity is 

 •0069. 



For oxygen, nitrogen, and carbonic oxide, the theory gives the conduc- 

 tivity equal to that of air. Hydrogen according to the theory should have 

 a conductivity seven times that of air, and carbonic acid about -J of air. 



III. On the means of increasing the Quantity of Electricity given 

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

 D.D. Received May 10, 1866. 



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 Leyden 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 towards increasing the power of the jar : this can- 

 not be done by increasing 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, how- 

 ever, is limited by the risk of destroying the acting surfaces of the rheo- 

 tome ; and by the decreasing rate at which the magnetism of the iron 

 core increases with the primary current. In some investigations 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^ 

 B + y 



in which is the product of the current and number of spires, A the 

 maximum lift of the magnet, and B the ^ which would excite it to half A. 



The rate of change ^ is therefore inversely as (B + '*I^)-. The results 



obtained with two of the magnets which I used will illustrate this. Their 

 A's are 781 lbs. and 278. 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 im- 

 plies 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 there- 

 fore not much to be gained in this direction. 



