THE INDUCTION OR ELECTRIC STRAIN 55 



Lanes and tubes of strain. A line, straight or curved, 

 drawn in the electric field so that at every point of its course the 

 tangent to it coincides in direction with the strain at the point is 

 termed a line of strain. We may suppose such a line traced out by 

 an " electric pointer " moved always in the direction of its length, 

 and with its positive end forward. Imagining the lines drawn 

 closely together and in all parts of the field, they map out the field 

 for us in regard to the direction of strain. There is no method 

 like that of the use of iron filings in magnetism which will show 

 the whole course of the lines, though sawdust scattered on a thin 

 ebonite plate gives some indication of the electric field. We may, 

 however, have a number of pointers in the field to show the 

 direction at different points. It is interesting to study in this way 

 the field due to the knobs of two or more charged Leyden jars. 

 \\ may show, for instance, that the field due to two similar jars 

 charged equally and oppositely from the terminals of an influence 

 machine resemble the magnetic field of a bar magnet. 



A bundle of lines of strain forms a tube of strain. 



Results deduced from the inverse square law. We may 

 at once applv to electric strain the results proved in Chapter III for 

 systems in uhirli the inverse square la\s holds. It i> important to 

 notice that tl? quantity whial v.- ha\e there called "intensity" 

 must be di\i'led by 4ir to ^i\e what uc hen- call strain, for the 

 intensitv just outside a conductor is I-T, while the strain is &. 



product strain X cross-see t ion i> constant for every 

 >\\ of a given tulie. Calling strain x cross-section 

 the total strain at the section, we may say that the total 

 strain is the same throughout the tube. , 



Unit tube. If a tube starts from +1 of electrification it is 



ied n unit tube. Jf the area on which that + 1 is spread is a, 



and the surface d- T. then <ra = l. But if D be the strain 



<uit>ide the surface, D = (r = -. Then just outside the surface 



a 



Da = l,nnd this product is constant along the tube. The total 

 strain in a unit tube is, therefore, equal to unity. 



A tube starting from a given quantity of positive electri- 

 fication either continues indefinitely or ends on an equal 

 quantity of negative electrification. 



I '>r example, each unit lx-<;ins on +1 and ends on 1. Then 

 the statement in Chapter II, p. i^.j, that equal quantities of the two 

 itions always induce each other is true not only of the 

 whoN- charge, but also of the separate elements. We may connect 

 each element of the one with an equal element of the other by a 

 tulx in. 



Appl\: '. ,\ closed surface and remembering 



that we ha\e to di\idc intensity by I-TT to give strain, 



