442 Prof. Thomson on the Mechanical Theory of Electrolysis. 

 i=15Vg-y*'' sin (^+20°) cos ^</^=15 x 1523 .^tt sin 20°. 



— IT 



If we take r=R, we find, from this, 



1=512,700,000,000, 



which is about 204000 times the intensity of a single cell of 

 Daniell's. One-half or one-thii'd of this amount would be the 

 electro-motive force experienced by a fixed circular conductor of 

 twice or three times the earth's diameter, at the time when the 

 earth's magnetic poles are passing under it. 



Greypoint, County Down, 

 October 6, 1851. 



Note on Electro-chemical Equivalents, 



The electro-chemical equivalents of zinc and hydrogen used in 

 the preceding paper were deduced from experiments made by 

 Mr. Joule on the electrolysis of sulphate of zinc and of water aci- 

 dulated with sulphuric acid, in which the galvanic currents used 

 were measured by means of a tangent galvanometer consisting 

 of a needle half an inch in length, suspended in the centre of a 

 cii'cular conductor one foot in diameter, fixed in the plane of 

 the magnetic meridian. The electro-chemical equivalent of a 

 substance, being defined as the mass (in grains) electrolysed 

 from any combination in a second of time by the action of 

 a current of unit strength, will be found by dividing the mass 

 of the substance electrolysed per second in any experiment 

 by the strength of the current. One way of combining several 

 experiments so as to obtain a mean result, will be to take 

 the arithmetical mean of the quantities of the substance found 

 to be electrolysed per second in the diiFerent experiments, and 

 divide it by the mean of the observed strengths of the cur- 

 rents. In the tangent galvanometer, the tangents of the angles 

 of deflection are proportional to the strengths of the currents, 

 and consequently the arithmetical mean of the tangents of the 

 angles of deflection in difi^erent experiments will be the tangent 

 of the angle of deflection corresponding to a current of mean 

 strength. The mean results, taken in this way, of four experi- 

 ments on the electrolysis of sulphate of zinc, and of four experi- 

 ments on the electrolysis of acidulated water, made at Manchester 

 on the 8th, 9th, 15th and IGth of September 1845, are as 

 follows : — 



Electrolysis of Sulphate of Zinc, 



Mean corrected tangent Zinc deposited 



of deflection. per second. 



•7345 -01508 grain. 



