WILLIAM SIEMENS, I-.R.S. 169 



Professor Sir William Thomson has produced a marine galvano- 

 meter, which is nearly independent, in its action, of external mag- 

 in t ic influences and of the disturbing influence of the ship's motion. 

 But these advantages are not realized without the sensitiveness of 

 the instrument being, to a very great extent, sacrificed. By mount- 

 ing the magnetic needle of the instrument upon a vertical spindle 

 ^ upon the end of a lever vibrating under the influence of a 

 hammer, I succeeded in obtaining greater sensitiveness, but 



the cost of a more complicated apparatus. 



THKORY OF DIFFERENTIAL MEASUREMENT. At this stage of 

 my inquiries, it occurred to me that both the resistance scales and 

 the galvanometer might be dispensed with in measuring electrical 

 resistances, by reverting to the principle of the voltameter in 

 combination with that of differential measurement. 



Faraday established the law that the decomposition of water in 

 a voltameter in an unit of time is a measure of the intensity of the 



V * 



current employed ; or, that I = - ; I being the intensity, V the 



volume, and t the time. 



According to Ohm's general law, the intensity, I, is directly 

 governed by the electro-motive force, E, and, inversely, by the 



E 



resistance, R, of the electric circuit, or, it is I = 77. 



E 



Combining the two laws, we have "V = r- t, which formula would 



R 



enable us to determine any unknown resistance, R, by the amount 

 of decomposition effected in a voltameter in a given time, and by 

 means of a battery of known electromotive force. 



Practically, however, such a result would be of no value, because 

 the electromotive force of the battery is counteracted by the polari- 

 zation, or electrical tension, set up between the electrodes of the 

 voltameter, which depends upon the temperature and concentra- 

 tion of the acid employed, and upon the condition of the platinum 

 surfaces composing the electrodes. The resistance to be measured 

 would, moreover, comprise that of the voltameter, which would 

 have to lie frequently ascertained by other methods, and the nota- 

 tion of time would involve considerable inconvenience and error. 

 For these reasons the voltameter has been hitherto discarded as a 

 measuring instrument, but the disturbing causes just enumerated 



