582-585] Electromagnetic Mass 509 



583. The value of C, as we have said, is equal to about 3 x 10 10 in 

 c. G. s. units. If units other than the centimetre, gramme and second are 

 taken, the value of C will be different. Since we have seen that C represents 

 a velocity, it is easy to obtain its value in any system of units. 



For instance a velocity 3xl0 10 in c.G. s. units = 6*71 xlO 8 miles per hour, so that if 

 miles and hours are taken as units the value of C will be 6'71 X 10 8 . 



584. The practical system of units is derived from the electromagnetic 

 system, each practical unit differing only from the corresponding electro- 

 magnetic unit by a certain power of ten, the power being selected so as to 

 make the unit of convenient size. The actual measures of the practical units 

 are as follows : 



Practical Units. 



Measure in 



Measure in electrostatic units 



Quantity Name of Unit electromag. units (Taking C = 3 x 10 10 ) 



Charge of Electricity Coulomb lO" 1 3 x 10 9 

 Electromotive Force } 



Electric Intensity I Volt 10 8 ^ 

 Potential 



Capacity Farad 10~ 9 9 x 10 n 



Microfarad 10~ 15 9 x 10 5 



Current Ampere 1Q- 1 3 x 10 9 



Resistance Ohm 1C 9 



9 x 10 11 



ELECTROMAGNETIC MASS. 



585. According to Maxwell's hypothesis of displacement-currents, which 

 as we have seen is justified by experiment, the motion of any charge of 

 electricity is accompanied by "displacement-currents" in the ether surround- 

 ing the charge. There is consequently a certain amount of electromagnetic 

 energy in the ether surrounding the moving charge : it has been found that 

 this energy must be regarded as kinetic. 



When a charged body, previously at rest, is set in motion, an electro- 

 magnetic disturbance is created in the immediate vicinity of the charges, and 

 this disturbance, as we have seen, will spread through the ether with a 

 velocity C, equal to that of light, or about 3 x 10 10 cms. per second. If the 

 charged body moves with a velocity which is small in comparison with the 

 velocity of light, we may, to an approximation, treat the velocity of light as 

 infinite, and this is equivalent to supposing that the whole field adjusts itself 

 instantaneously to the motion of the charged body. In such a case we shall 

 have a displacement-current at every point in the ether, of which the in- 

 tensity will be proportional to the velocity of the charged body, and will 

 depend in addition on the amount and distribution of the charge on the body. 



