1ELEGRAPHIC APPARATUS, 
219 
inside and outside of this Ley den-jar ; by putting b to earth 
or connecting it with the pole of the battery, an electro-static 
charge can be given to A, the quantity of which depends upon 
the number of cells in the battery and the extent of surface of 
the condenser. This last depends upon the number of sheets 
of the series that are opposed to each other. Condensers may 
therefore be made of any capacity, giving charges varying from 
that which a mile of cable is capable of receiving up to that 
of one of the trans-Atlantic cables. The unit or standard by 
which these capacities are measured is called the microfarad , 
and a condenser of this capacity takes a charge equal to that 
