268 ELEMENTARY LESSONS ON [CHAP, v. 



the " strength " of the shell will be equal to the 

 thickness of the shell multiplied by the surface- 

 density. 



312. Intensity of Field. We have seen (Art. 105) 

 that every magnet is surrounded by a certain " field," 

 within which magnetic force is observable. We may 

 completely specify the properties of the field at any 

 point by measuring the strength and the direction of that 

 force, that is, by measuring the " intensity of the 



field " and the direction of the lines of force. We might 

 take as a measure of the intensity of the field* at any point 

 the number of lines of force that pass through the region 

 about that point ; but for the present we will define it as 

 follows: The "intensity of the field" at any point is 

 measured by the force with which it acts on a unit 

 magnetic pole placed at that point. Hence, unit intensity 

 of field is that intensity of field which acts on a unit pole 

 with a force of one dyne. There is therefore a field of 

 unit intensity at a point one centimetre distant from 

 the pole of a magnet of unit strength. Suppose a 

 magnet pole, whose strength is ;;z, placed in a field at a 

 point where the intensity is H, then the force will be ;;/ 

 times as great as if the pole were of unit strength, and 

 / == m x H. 



313. Intensity of Magnetisation. When a piece 

 of a magnetic metal is placed in a magnetic field, some 

 of the lines of force run through it and magnetise it. 

 The intensity of its magnetisation will depend upon the 

 intensity of the field into which it is put, and upon the 

 metal itself. A metal in which, like soft iron, a high 

 degree of magnetisation is thus produced is said to 

 possess a high coefficient of magnetisation. Every 

 magnetic substance has a positive coefficient of mag- 

 netisation ; but there are many substances, such as 

 bismuth, copper, water, etc., which possess negative 

 coefficients of magnetisation. The latter are termed 



