10 Mr. Barlow on the Efficiency of the Galvanometer 



The following table shows the comparative conducting 

 powers of different metals according to the determination of 

 Sir Humphry Davy and M. Becquerel, the power of copper 

 in each being called 100. 



Relative conducting power of 

 different Metals according 

 to Sir Humphry Davy. 



Copper 100 



Silver 109 



Gold 73 



Lead 69 



Platinum 18 



Iron 14*5 



Relative conducting power of 

 different Metals according to 

 M. Becquerel. 



Copper 100 



Silver 73 



Gold 93 



Lead 8*3 



Platinum 16*4 



Iron 15-8 



The names of these authors are a sufficient guarantee that 

 each has reported his results correctly, and there must be 

 therefore some cause for the discrepancies involved in their 

 estimations which it is very desirable to discover ; for till this 

 is done we cannot be said to be in possession of any law of 

 conduction in electricity derived from one source only, and 

 much less are we enabled to assign the comparative conduc- 

 ting powers when derived from different sources. 



According to the idea I have advanced, the galvanometer 

 only measures the conducting power of a metal while the wire 

 is so small as to be insufficient to carry off the generated elec- 

 tricity ; beyond that point it only measures the intensity of the 

 battery. Now this intensity certainly varies with the length 

 of the conductor. Sir Humphry Davy, it appears, considered 

 the law to be inversely as the length, and M. Becquerel states 

 this law distinctly ; while the experiments I have referred to 

 make it nearly inversely as the square roots of the lengths. 

 So that while M. Becquerel, by quadrupling the length of his 

 wires, reduced his deflections to about one fourth, mine were 

 only reduced to about a half, and with nine times the length 

 to about one third. 



My battery, as I have shown, had a large generating sur- 

 face ; what the other batteries may have been I cannot tell ; 

 but if, according to the view I have taken, my battery gene- 

 rated the fluid faster than the conductor could carry it off, 

 and the others did not, we see at once why the intensity in 

 my instrument did not exhibit the same reduction ; but if 

 from the nature and magnitude of these batteries this cannot 

 be supposed, then this explanation fails and some other must 

 be sought for. 



When we reflect that besides this great discrepancy in the 

 law of the length, we have a still greater in that which re- 



