232 



Prof. G. C. Foster's Description of some 



portional to the difference of potential E between the insulated 

 conductor and the source, and to a coefficient S called the elec- 

 tric capacity of the conductor and depending on the extent and 

 disposition of its surface, and its position relatively to other con- 

 ductors. This relation is very easily proved by means of a 

 Thomson's galvanometer connected with a Leyden battery and 

 a galvanic battery in the way shown in the figure. 



G=galvanometer. 

 B=galvanic battery. 

 C = Leyden battery. 

 K=key. 



E=eartb-contact. 



For example, a Leyden battery of six jars, each jar having a 

 diameter of 18 centims. and being coated to a height of 24 cen- 

 time, from the bottom, was charged and discharged through the 

 galvanometer by four Grove's cells arranged in series. The sum 

 of the deflections on both sides of zero, due to the charge and 

 discharge, was (as the mean of several experiments) 88*8, the 

 highest reading being 90, the lowest 88. When three of the jars 

 were removed, so as to leave a battery of only half the previous 

 capacity, the mean reading of several experiments was 45*1, the 

 maximum being 45*5 and the minimum 44*5. 



3. Comparative Measurement of Electromotive Force. — Pre- 

 cisely the same arrangement of apparatus and mode of experi- 

 menting that serves for comparing the capacities of conductors, 

 also serves for comparing the electromotive forces of batteries; 

 but, in order to make the comparison more accurate, it is ad- 

 visable to substitute a conductor of greater capacity for the Ley- 

 den battery mentioned in the last paragraph, unless the electro- 

 motive forces to be compared are rather considerable. In the 

 following experiments the condenser of a medium-sized Ladd's 

 induction-coil was used. 



When the condenser was charged and discharged through the 

 galvanometer by one Grove's cell, the sum of the readings on 



