CHAP, v.] ELECTRICITY AND MAGNETISM. 287 



rent and the strength of the electromagnet it produces, the 

 following approximate rule : The strength of an electro- 

 magnet is proportional to the angle whose tangent is the 

 strength of the magnetising current ; or 



where i is the current strength and A a constant 

 depending on the construction of the particular magnet. 

 If the student will look at Fig. 90 and imagine the 

 divisions of the horizontal line OT to represent strengths 

 of current, and the number of degrees of arc intercepted 

 by the oblique lines to represent strengths of magnetism, 

 he will see that even if OT be made infinitely long, the 

 intercepted angle can never be greater than the fixed 

 limit of 90. 



(b) The strength of an electromagnet is proportional to 

 the number of turns of wire in its coils. This also is 

 only true when the iron core is far below saturation ; and 

 it is only true when the current is kept constant. For 

 if by putting on more coils of wire we add materially td 

 the total resistance of the circuit, the strength of the 

 current will, according to Ohm's Law (see Arts. 1 80 and 

 345), be thereby reduced. This has an important 

 bearing on the construction of telegraphic and other 

 instruments ; for while electromagnets with " long coils," 

 consisting of many turns of fine wire, must be used on 

 long circuits where there is great resistance, such an 

 instrument would be of no service in a circuit of very 

 small resistance, for the resistance of a long thin coil 

 would be disproportionately great : here a short coil of 

 few turns of stout wire would be wanted. 



(c) The strength of an electromagnet is independent of 

 the thickness and material of the conducting wire. 

 The wire may be of any metal of any thickness, pro- 

 vided only it carries enough current a sufficient number 

 of times round the core to produce a field of the requisite 

 strength. 



