MOTIONS OF ELECTRONS 177 



proach one at a time instead of both at once frequently 

 leads to a neglect of the fundamental realities and hence 

 to a treatment of each subject as a group of apparently 

 unrelated phenomena. 



To designate the portions of the two subjects which 

 are of importance to students of both a new name has 

 come into use, that of Physical Chemistry. This treats 

 of the portions of chemistry where energy relations, the 

 behavior of molecules, and then- electronic composi- 

 tion, are most evidently involved. As yet, however, 

 this name and the unified treatment of these funda- 

 mental relations has been reserved for the occasional 

 advanced student. In elementary treatments the sub- 

 jects are still separated. 



The phenomena of which Volta's batteries are an 

 illustration classify under the title of physical chem- 

 istry. From the standpoint, however, of the engineer, 

 who is interested in effects rather than causes, Volta's 

 contribution to science may be described as the dis- 

 covery of a source of continuous current at a low but 

 fairly constant potential difference. 



By current we mean the time rate at which electricity 

 is transferred, that is, the number of electrons per sec- 

 ond which move across any section of the conducting 

 path. Thus if the zinc and copper plates of a voltaic 

 cell are connected by a wire a stream of electrons flows 

 through this wire from the negative zinc to the positive 

 copper plate. Whatever units we use for measuring 

 the quantity of electricity, unit current will be flowing 

 in the wire when electrons are transferred at the rate 

 of one unit of quantity per second. For example, using 

 unit quantity as defined by reference to Coulomb's 



