PRELIMINARY NOTIONS. 7 



causes us to remain in the field of hypotheses, and to designate 

 the initial causes by certain names without being able to define 

 them with absolute accuracy. It is, therefore, important that 

 the new designations should be made well known by comparing 

 them \\ith the physical, mechanical, and chemical phenomena 

 which are more generally understood. 



For instance, the special property of electricity which 

 corresponds to the expanding force of gases, to the hydrostatic 

 pressure of liquids, to the temperature of bodies in thermo- 

 dynamics, is called electric potential. It is owing to the 

 difference of potential in two points, that electricity will travel 

 from one to the other. Two bodies are said to have the same 

 potential when, on being connected by a conductor, no electric 

 motion takes place.* 



A difference of potential, like a difference of level, is in- 

 dicative of a condition of relation between two points without 

 involving the necessity of ascertaining the exact value of the 

 potential at those points. In the same manner as hydraulic 

 work is calculated by multiplying the weight of the water by 

 the head of the fall, so is electrical work calculated by multi- 

 plying the quantity of electricity in motion by the difference 

 of potential. 



Two conducting bodies joined by a metallic wire become one 

 single conductor and set at the same potential. The earth may 

 be considered as an immense conductor the potential of which 

 is nil : an electrified body therefore sets at a nil potential when 

 it is put in communication with the ground. 



The electric capacity of an insulated conductor is the quantity 

 of electricity which is required to raise its potential by one unit. 



The electric quantity is measured by the power of attraction 

 developed by the current. If a quantity A of electricity exerts 

 a force double that of a quantity B, under the same conditions, 

 the quantity A is said to be double the quantity B. 



ELECTROMOTIVE FORCE. The electromotive force is that 

 particular force, whatever its origin, which tends to produce a 

 displacement of electricity. The difference of potential between 

 two points gives the measure of the electromotive force. 



* ' DOS Grandeurs electriqucs,' by E. E. Blavier. 



