Prof. F. Guthrie on a new form of Voltameter and Voltastat. 335 



extremely small as not to allow the mercury, when at the top 

 of D, to fall in less than ten minutes or a quarter of an hour. 

 The cork and top of the tube C are made perfectly airtight by 

 sealing-wax. 



On connecting H and I with the poles of a battery, the water 

 in C is decomposed ; the detonating gas can only escape by the 

 opening F. The hindrance it meets with on account of the 

 small size of F causes it to force the mercury up D. The effect 

 of the rise of the mercury in D is twofold ; it forces more gas 

 through F, at the same time that it lays bare a part of the elec- 

 trodes, leaving the active portions of them further apart. The 

 quantity of gas produced is accordingly diminished. In other 

 words, an increase in the strength of the current produces effects 

 which diminish the strength of the current. The instrument is 

 a regulator, which may be compared with the governor of a steam- 

 engine. 



It is found that the mercury rises to a certain height in D, 

 and remains nearly at that height for many hours. Since a cer- 

 tain height of the mercury in D corresponds to a certain pres- 

 sure in the interior, and since the latter corresponds to a certain 

 rate of delivery through F, it follows that the quantity of elec- 

 tricity passing through the instrument is directly measured by 

 the height of the mercury. The actual relation between the 

 height of the mercury and the electricity which passes may, of 

 course, be measured either by interposing a galvanometer or 

 actually measuring the quantity of gas evolved by fitting a gas- 

 delivery tube over F, &c. 



Nevertheless an exceeding gradual depression in D must occur 

 even with a perfectly constant current, on account of the removal 

 of a portion of the liquid C by electrolysis. For many purposes 

 this is negligible, namely when the instrument is used as a vol- 

 tastat. It is clear that, as a certain height in D corresponds to 

 a certain activity, the amount of water lost from C in a given 

 time is measured by the height in D. Hence, on knowing the 

 relative diameters of D and A, and knowing the amount of water 

 decomposed in a given time, a simple calculation serves to make 

 the minute correction. 



London Institution, 

 April 3, 1868. 



