NEW FORM OF VOLTAMETER. 189 



making investigations on the solar phenomena. These experiments 

 of Dr. 'sGravesande's are very remarkable, and show what a great 

 philosopher he was. At all events, when he found himself in error, 

 he was only rejoiced to have discovered the cause and to give credit 

 to Leibnitz for the correctness of his theory. I have now only to ask 

 you to return your thanks again to Dr. Rijke. 



ON A NEW FORM OF VOLTAMETER. 



Baron FERDINAND DE WRANGELL : The instrument to which I 

 have the honour to draw your attention has been devised by my 

 friend Professor Robert Lenz of St. Petersburg, and as he is unable 

 to attend he has requested me to explain the principle on which it is 

 founded. I will not go into the details now, but if any one is interested 

 in the subject I shall be very glad to explain the construction of the 

 instrument in its details at any time. The instrument is intended for 

 volta-metric measurement of the strength of a current, and it is based 

 upon the principle that the quantity of matter decomposed by the 

 current which flows through it is proportional to the strength of the 

 current, all other conditions remaining equal. This assumption that 

 all other conditions remain equal is the most important point and 

 the most difficult to attain in a voltameter. Besides this first con- 

 dition of equality of conditions from one measurement to another, in 

 order to compare the two currents, of course brevity of time and ease 

 of management are very important conditions, and I think that this 

 mercury voltameter fulfils all these conditions in a much higher 

 degree than those which are generally used. The best mode of 

 which I am aware consists in measuring the decomposition of a salt 

 of silver solution by a current. The solution is contained in a 

 platinum vessel, and after the decomposition has taken place for a 

 certain time, the vessel is washed out, then dried perfectly, and the 

 amount of silver which has been decomposed by the current in a 

 certain period of time is ascertained by weighing. This process of 

 washing out the silver, secondly, of drying it, and thirdly, of weighing 

 it by a chemical balance, takes a great deal of time, is very trouble- 

 some, and is liable to some errors, of which the most important is 



