330 Dr. Tyndall on the Progress of the Physical Sciences. 



described below were determined by direct experiment. The 

 electromotive force was also determined, the reduced length of 

 the circuit was found by measuring the resistances of its various 

 parts, and from these two, the electromotive force and the re- 

 duced length, the tensions due to the same points were calculated 

 by the foregoing formula. 



Points examined. 



a. The second lower angle of the zigzag. 



b. The fourth lower angle of the zigzag. 



c. The sixth lower angle of the zigzag. 



d. The point where the zigzag joined the copper. 



e. The solution of sulphate of copper 202 inches from the 

 plate of copper. 



/. The solution of sulphate of copper 4"02 inches from the 

 plate of copper. 



g. The solution of the sulphate of copper 6 inches from the 

 plate of copper. 



h. The solution of sulphate of copper 8 inches from the plate 

 of copper. 



In the following table the results obtained by calculation are 

 compared with those obtained by direct experiment ; the quan- 

 tity \ is the same as that contained in the formula. 



The truth of Ohm's formula, which he derived from consi- 

 derations purely theoretical, appears to be placed beyond the pale 

 of doubt by these results. Hitherto the celebrated law which 

 usually bears his name has rested upon a basis of conjecture 

 merely ; and to the extraordinary patience and refined experi- 

 mental skill of M. Kohlrausch is due the credit of giving to this 

 conjectural foundation the stability of fact. 



It may be stated, in addition, that the same physicist has also 

 examined the thermo-circuit, and has not only demonstrated the 

 existence of electric tension at its poles, but also proved that the 

 electricity obeys the same law of distribution as that true for the 

 voltaic circuit. 



Queenwood College. Marci 1852. 



