346 On the relation of Common and Voltaic Electricity. 



mical action of a grain of water upon four grains of zinc can 

 evolve electricity equal in quantity to that of a powerful thun- 

 der-storm." This was written in 1833, when the application of 

 a mechanical standard or work equivalent to molecular forces 

 was but little thought of. To avoid conveying an incorrect im- 

 pression now, it would be necessary to underline the word 

 "quantity," and add, "but of incomparably less intensity than 

 that of a powerful thunder-storm." The idea of the mechanical 

 equivalent of a grain of water being equal to the mechanical 

 equivalent of a thunder-storm would thus be excluded. 



The progress of science, and the labours of Harris especially, 

 has since enabled us to obtain some clearer ideas of quantity and 

 intensity as applied to electricity. When the mechanical value 

 of a constant quantity under different degrees of intensity has 

 been ascertained — and this seems practicable with Harris's appa- 

 ratus — we shall be in a position to estimate exactly the potential 

 relation between voltaic and common electricity. In the meaii 

 time it may be useful to direct attention to certain data which 

 already exist, by means of which we may roughly calculate an 

 approximate result. 



The great electric battery of the celebrated Dutch electrician, 

 Van Merum, consisted of 100 jars, each exposing 5^ square feet 

 of coated glass, making altogether 550 square feet. It is stated 

 that this battery, discharged through a length of 25 feet of iron 

 wire yl^oth of an inch in diameter, fused it so that it was con- 

 verted into red-hot balls thrown in all directions. Assuming 

 that the heat evolved was sufficient to raise the temperature of 

 the wire 3000 degrees, we have j^ of a cubic inch of iron thus 

 heated ; this is equivalent to about -^-g cubic inch of water raised 

 3000 degrees, or 15,000 grains raised 1 degree. 



To compare this battery strictly with that employed by Fara- 

 day, we should require to know the electric tension of each when 

 charged, as indicated by the same electrometer ; also the thickness 

 of the glass in each. Such data are wanting ; but, for a rough 

 estimate, we may perhaps assume that they did not differ mate- 

 rially in these particulars. 



Faraday states that the quantity of electricity required to de- 

 compose a single grain of water is equal to 800,000 charges of 

 a 25 square feet battery, each charge made by thirty turns of a 

 plate-glass machine, 50 inches diameter, in full action. The 

 product of 25 by 800,000 is 20,000,000 square feet of coated 

 glass. This, compared with 550, shows that the quantity of 

 electricity associated with 1 grain of water is upwards of 36,000 

 times the amount in Van Merum's battery, and consequently 

 "the heating power must be equivalent to 15,000 grains of water 

 raised 36,000 degrees. 



