174 THE REALITIES OF MODERN SCIENCE 



The early study of electricity followed two lines be- 

 cause the investigators wrongly distinguished between 

 " fractional" electricity and " galvanic" electricity. 

 Electrification produced by friction had, of course, 

 long been recognized. The quantitative law for the 

 action of two charges produced in this way was stated 

 in 1785 by Coulomb in a form similar to Newton's 

 law of gravitation. The force acting between two 

 charged bodies is proportional to their charges and 

 inversely as the square of the distance between cen- 

 ters. This law leads to a unit for electricity since it 

 may be written as 



(1) 



where qi and q 2 are the two quantities of electricity, F 

 is the force, r is the distance (between centers), and K 

 is a factor of proportionality depending upon the 

 choice of units and upon the medium. We must select 

 some medium, as for example, air, and let K be unity 

 for this condition. Applying to equation (1) the 

 method of Chapter X, it appears that unit electricity 

 is such a quantity that when placed in air at a distance 

 of 1 cm. from an equal quantity it will repel it with a 

 force of 1 dyne. This is now known as the electro- 

 static unit of electricity. 



About 1780, Galvani, professor of anatomy in 

 Bologna, observed a peculiar phenomenon in connec- 

 tion with the legs of some newly skinned frogs which 

 were awaiting his examination. In the room there 

 was a machine for producing electrification continu- 

 ously by the friction of two dissimilar substances. 

 When a sufficient charge had been accumulated a spark 



