104 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



meters of the portion of the steel rod over which the pole is uni- 

 formly distributed. The non-uniformity of the field near the 

 ends of the slim pole is negligible if r is small in comparison 

 with /. The effect of this slim pole is therefore to produce a 

 radial magnetic field over the whole of a portion of the coil of 

 length /. This portion of the coil contains Iz turns of wire, and 

 the length of each turn is 2irr so that the total length of wire in 

 the region where field is produced by the slim pole is 2irrlz. 

 This wire is everywhere at right angles to the field H 1 (which is 

 due to the slim pole) and it is therefore pushed sidewise by a force 

 F= /x 2Trrlz x //', or, using 2mjrl for H' , we have 



F= Arirzlm 



but the force with which the slim pole pushes on the coil is equal 

 and opposite to the force with which the coil pushes on the pole, 

 and the force with which the coil pushes on the pole is equal to 

 the product of the strength of the pole and the field intensity at 



the pole due to the coil. There- 

 fore the field intensity inside of 

 the coil is equal to \trzL 



57. The tangent galvanometer. 

 One of the earliest forms of 

 instrument for measuring the 

 strength of the electric current 

 was the tangent galvanometer. 

 It consists essentially of a circu- 

 lar coil of wire at the center of 

 which a small magnet is sus- 

 pended, as shown in Fig. 690. 

 This suspended magnet carries a 

 pointer which plays over a di- 

 vided circle by means of which the angle through which the 

 magnet is turned when a current is sent through the wire may 

 be observed. The coil of wire is mounted with its plane vertical 

 and magnetic north and south. 



Fie- 69a. 



