508 Displacement Currents [OH. xvn 



So also from the unit magnetic pole can be derived other units e.g. of 

 magnetic force, of magnetic potential, of strength of a magnetic shell, etc. 

 in which to measure quantities which occur in magnetic phenomena. These 

 units will belong to the electromagnetic system. 



If electric phenomena were entirely dissociated from magnetic phenomena, 

 the two entirely different sets of units would be necessary, and there could be 

 no connection between them. But the discovery of the connection between 

 electric currents and magnetic forces enables us at once to form a connection 

 between the two sets of units. It enables us to measure electric quantities 

 e.g. the strength of a current in electromagnetic units, and conversely we 

 can measure magnetic quantities in electrostatic units. 



We find, for instance, that a magnetic shell of unit strength (in electro- 

 magnetic measure) produces the same field as a current of certain strength. 

 We accordingly take the strength of this current to be unity in electro- 

 magnetic measure, and so obtain an electromagnetic unit of electric current. 

 We find, as a matter of experiment, that this unit is not the same as the 

 electrostatic unit of current, and therefore denote its measure in electro- 

 static units of current by (7. This is the same as taking the electromagnetic 

 unit of charge to be C times the electrostatic unit, for current is measured 

 in either system of units as a charge of electricity per unit time. 



In the same way we can proceed to connect the other units in the two 

 systems. For instance, the electromagnetic unit of electric intensity will be 

 the intensity in a field in which an electromagnetic unit of charge experiences 

 a force of one dyne. An electrostatic unit of charge in the same field 



would of course experience a force of ^ dynes, so that the electrostatic 



measure of the intensity in this field would be ^. Thus the electro- 







magnetic unit of intensity is ~ times the electrostatic. 



The following table of the ratios of the units can be constructed in 

 this way : 



Ratios of Units. 



Charge of Electricity. One electromag. unit = G electrostat. units. 



Electromotive Force. = l/C 



Electric Intensity. = l/C 



Potential. =1/0 



Capacity. = C* 



Current. =C 



Resistance of a conductor. = l/C 2 



Strength of magnetic pole. = I/O 



Magnetic Intensity. = C 



