316 THE ELECTRIC TELEG11APH. 



if the whole resistance of the circuit be = 1, the electro- 

 motive force will be equal to the intensity, which is also equal 

 to the number of cubic centimeters of gas developed in a 

 minute. The unit of resistance in this case has been taken 

 as that of a prism of copper, 1 millimeter section, and 1 meter 

 long. 



According to this method of determination, the electro- 

 motive forces of the following elements are : 



Zinc Carbon element Deleuil =839 

 Ditto ditto Stohrer = 777 



Zinc Platinum ditto Grove =829 

 Zinc Copper ditto Daniell = 470 



Or, were the resistance of the circuit that of a copper prism 

 of the above dimensions, each of these elements would 

 decompose so many cubic centimetres of gas in one minute, 

 as is indicated by the number set opposite to it in the list. 



56. Another method of measuring the electro-motive force 

 is by means of the unit adopted by Eegnault, namely, the 

 electro-motive force of a thermo-electric pair of copper- 

 bismuth wires, whose soldered ends are kept at the constant 

 temperatures of C., and 100 C. To measure the electro- 

 motive force of any element, Eegnault combines a number of 

 these copper-bismuth elements together, until, when the 

 currents of the two batteries are opposed, they exactly com- 

 pensate each other. The number of thermo- elements required 

 to do this is, therefore, the measure of the force of the 

 element under inquiry ; and, as the force of each individual 

 thermo- element is very small, the number brought to balance 

 a galvanic element is great enough to allow of very nice 

 adjustment, and would give very good results if the electro- 

 motive forces of 'the individual thermo -elements were practi- 

 cally equal, which unhappily is not the case. 



The electro-motive force of a DanielFs element, whose 

 copper-plate is immersed in a concentrated solution of sul- 

 phate of copper, and whose zinc is immersed in dilute 

 sulphuric acid of the strength of 1 part, by weight, of acid, 



