288 LECTURES TO SCIENCE TEACHERS. 



communicating with each other by a narrow tube, the 

 mercury in both vessels being covered with a solution of 

 nitrate. The current passing causes more mercury to be 

 dissolved in one vessel, and more mercury to be separated 

 from the solution in the other, so that there is a continual 

 carrying of mercury from one vessel to the other, by the 

 action of the current. The rate of increase in the lower 

 vessel, or, in other words, the quantity carried over in 

 a given time measures the strength of the current. The 

 apparatus is so arranged as to give the means of observing 

 with very great accuracy the rate of increase in the 

 quantity of mercury. There is a micrometer screw, by 

 means of which the surface of the mercury can be lowered, 

 and read off exactly against a mark, and then, after the 

 experiment, the same measurement is made again, and the 

 difference between the two readings of the screw indicates 

 the increase in the volume of mercury. It is said that in 

 the course of a minute or two a sufficient quantity of 

 mercury is carried over to give a measurable increase, 

 and that the measurement can be made with very great 

 accuracy indeed. The essential electro-chemical principles 

 involved are the same as in the ordinary voltameter, but 

 there is a different mechanical arrangement for observing 

 the amount of effect produced. 



I have only time to state very briefly the general principles 

 of the methods available for measuring Electromotive Force. 

 In the most usual cases electric currents are produced 

 either by chemical action or by the relative motion of 

 magnets and conductors. In an ordinary galvanic battery 

 we have chemical action taking place as long as the current 

 passes, the quantity of the current which traverses the 

 circuit being proportional to the quantity of chemical action 

 which takes place. In apparatus, such as this on the table, 

 we have currents produced by the motion of magnets and 

 conducting-wires relatively to each other. An absolute 

 measure of electro-motive force may be derived from a study 

 of the conditions under which currents are produced in 

 either of these cases. 



Thus, first, if we can determine the amount of energy 

 expended in a galvanic battery when a unit of electricity 

 traverses the circuit, we have really calculated the electro- 

 motive force. Now the quantity of heat corresponding to 



