ELECTRODYNAMIC UNIT OF INTENSITY. 459 



473. ELECTRODYNAMIC UNIT OF INTENSITY. If, with Ampere, 

 we directly make h=i in formula (13), the strength of the current 

 will be expressed as a function of a particular unit, which is called 

 the electrodynamic unit. 



This unit will be defined by the formula itself. By making 



e=0, 



ds = ds' = i , 

 i>fVi, 



we get 



In this case the currents are parallel, of unit length, perpen- 

 dicular to the line which joins their centres, and at unit distance j 

 the strength of the current, which is equal for each of them, and 

 is taken at unity, is such that the reciprocal action is equal to the 

 unit of force. 



Supposing the currents equal, equation (14) will give, 



The electrodynamic intensity of a current is equal to its electro- 

 magnetic intensity multiplied by \/2. 



In virtue of the ratio which connects the numerical expression 

 of a magnitude into the unit which serves to measure it, we see that 

 the electrodynamic unit of current is equal to the electromagnetic unit 

 divided by \/2. 



474. The identity between the mutual action of currents and 

 that of the correlated magnetic systems has been confirmed in all 

 experiments as long, at least, as a steady condition has been estab- 

 lished in the circuits. 



We may cite, for instance, the experiments of Weber on the 

 reciprocal action of the cylindrical coils with circular bases. This 

 action is proportional to the strength of the two currents ; it varies 

 with the relative distance and direction of the coils according to 

 the same law as that of two magnets whose axes are respectively 

 parallel to the axes of the coils. 



