Permanent Magnetic Field. 309 



vertical section of the magnetic circuit. In this, N S is a 

 cylindrical steel rod, 2£ inches long and 1 inch in diameter, 

 attached to two cast-iron pole-pieces. The upper pole-piece, 

 a a, is a circular disk, 4 inches in diameter and § inch thick, 

 carefully bored to fit the upper end of N S. The lower pole- 

 piece, bb, is nearly hemispherical, and about \ inch thick in 

 the wall. This is attached to the steel rod by means of an 

 iron pin, let into N S and screwed into b b. 



Fig. 2. 



The opposed faces of the pole-pieces are carefully turned in 

 the lathe, so as to leave between them a circular air-gap less 

 than -^q inch wide. 



The above description gives the form and dimensions of the 

 magnetic part of the instrument. It is magnetized after being 

 put together by a current sent through a coil wound on the 

 steel rod. 



The other parts consist of : — 



(1) Mechanical arrangements which will rigidly preserve 

 the pole-pieces iu position. 



(2) Arrangements which will carry and guide a coil of 

 wire as it is thrust through the field in the very narrow air- 

 gap. Screwed to the upper surface of the disk a a is a brass 

 casting with three projecting lugs 11 (fig. 1). These lugs are 

 screwed both to small blocks attached to the hemispherical 

 pole-piece, and also to the tops of three brass pillars (fig. 1). 



The coil is wound in a shallow groove cut on the outside of 

 a brass ring, which is turned thin enough in the wall to slide 

 freely through the air-gap. Attached to its upper edge are 

 three arms which radiate from a central boss, the latter 

 sliding up or down on the central guide-rod a (fig. 1). The 

 radial arms support an ebonite disk on which are fixed the 

 terminals of the coil. 



