50 STATIC ELECTRICITY 



the insulator, it is usual to call it the Dielectric. The region round 

 a system in which electric strain is manifested is termed the Held 

 of the system. 



Even though at present we can give no complete explanation 

 of electric strain, we can still compare the magnitude of the strain 

 at different points of the field by means of some chosen effect due 

 to it, and we may also assign direction to it. For instance, the 

 magnitude may be taken as proportional to the force on a \ 

 small charged body, the direction coinciding with that of the t 

 exerted. Or some other effect may be taken, such as the charge 

 which gathers on either side of a proof plane when held perpen- 

 dicularly to the direction of the force. 



Similarly, without attempting to explain the nature of a beam 

 of light when traversing a transparent medium, we may assign 

 direction to it at any point say that of the normal to a sir. 

 held in the position of maximum illumination and we may com- 

 pare the magnitude at different points by the illumination of an 

 interposed surface. 



The direction of electric strain. Ix?t us suppose that 

 a small insulated conducting sphere is placed at any point in tin- 

 field. Then one hemisphere will be positively, the other nega- 

 tively electrified. This we know from direct experiment. Ti 

 will be one diameter about which each charge is symmetrically 

 arranged if the sphere is sufficiently small, its ends being at the 

 two pointsof maximum electrification. We may define the direction 

 of electric strain as that of the diameter so drawn, and ue shall 

 consider it as drawn from the negative towards the posit i\ r 

 electrification. 



But we shall have similar symmetrical eh edification at the two 

 ends of any small conductor itself symmetrical about an axis and 

 an equatorial plane, when the axis i 

 parallel to the direction of strain as al> 

 J, ^2* defined. For instance, if two proof \ 



be held together in the position shoun in 

 Fig. 40, the arrow-head denoting the d 

 tion of strain, the one is entirely posit 

 the other entirely negatively electrified. 

 FIG. 40. on separating the two while in this portion 



the charges may be shown to be equal 



opposite by the electroscope. Further, the charge on each plane is a 

 maximum when so held. For on holding the planes at some other 

 angle to the strain, and then separating and testing the charges, 

 these will be found appreciably less than when held as in Fig. 40. 

 When the planes are as in Fig. 41 the electrification is as indi- 

 cated, and on separating and testing the planes they are found to 

 be unelectrified. From this effect of induction, combined with the 

 tendency of a positively electrified body to move with, and a nega- 

 tively electrified body to move against, the strain, we may obtain a 



