422 



Various coils of wire may be used, according to the quantity 

 of the currents to be measured. With a coil of very fine 

 wire the instrument is moderately sensible, and is applicable 

 when the currents are too powerful to be estimated by the 

 torsion galvanometer. 



I believe the instrument possesses the following advantages : 



That the indications are by weight, being a direct ratio to 

 the forces of the currents. 



That various coils of known lengths and thicknesses of 

 wire can be used, and can be readily changed. 



That by using short and thick wires it may be employed 

 for very powerful currents, with little danger of disturbing 

 the polarity of the magnet. 



That the magnet being fixed, the indications are not dis- 

 turbed by the attraction of iron apparatus or of the con- 

 ducting wires. 



That the apparatus is easily constructed, and does not 

 require delicate workmanship. 



It will also be readily seen that the apparatus may be 

 employed for ascertaining the relative strength of horse-shoe 

 magnets. 



After finding that the instrument worked very satisfactorily 

 in all other respects, I endeavoured to ascertain whether the 

 weights counterbalanced might be regarded as strictly ex- 

 pressing the force or dynamic eff'ect of the voltaic current. 

 For this purpose I had recourse to the method pointed out 

 by Professor Wheatstone, in his admirable essay, printed in 

 the Transactions of the Royal Society for 1843, paragraph 

 18, page 326. 



Feeling some little doubt, however, as to the correctness 

 of Ohm's theory, and of Professor Wheatstone's application 

 thereof, on the ground that it appears to me that the resist- 

 ance of a conductor is not, in all cases, in the direct ratio of 

 its length, I thought it preferable to make use of another 



