25-1 HENEY A. EOWLAND 



system is made by fitting two small square blocks of wood on an alumi- 

 nium wire; on two sides of each of the wooden blocks are cemented 

 small scraps of highly magnetized watch-spring. The needle thus made 

 is about 1 X 1 X 10 mm. 



The mirror is fixed just below the upper needle, and is read by a 

 telescope 200 cm. distant. The plane of the mirror is at an angle 

 of 45 with the plane of the disks for convenience. The whole is sup- 

 ported by the board 00 attached to a wall -bracket. 



Two controlling magnets (W W) with their poles turned in opposite 

 directions are used. By means of the up and down motion of either 

 magnet, any change in the sensitiveness can be attained; and by the 

 motion in azimuth, the zero point is controlled. The advantage of its 

 use lies in the extremely delicate means it affords of changing the 

 sensitiveness, much more delicate than with a single magnet. 



The bed-plate B is screwed to one end of a table, at the other end of 

 which a countershaft is placed (Fig. 2). This is run by an electric 

 motor in the next room, the belt running through the open doorway. 

 The motor is 14 metres from the needle. 



Although the disks and countershaft were carefully balanced when 

 first set up, and the table braced and weighted by a heavy stone slab, 

 yet at the speed used, 125 per second, the shaking of the entire appar- 

 atus was considerable; the needle was so unsteady that it could not be 

 read. This was seen to be due to vibrations of the telescope itself and 

 not to the needle. To prevent it, each leg of the table on which the 

 telescope rested was set in a box about 30 cm. deep filled with saw- 

 dust, and a heavy stone slab was placed on top of this table. This 

 entirely did away with the trouble; the swing of the needle was as 

 regular when the apparatus was revolving as when it was at rest. 



The two hard rubber rings (RR) mentioned above have grooves cut 

 in their peripheries ; in these grooves wires are wound. These serve as 

 a galvanometer for determining the needle-constant. When not in use 

 they are held in the position shown in the figure, but when it is desired 

 to determine the needle-constant they are slipped on the shoulders 

 (AAAA) and pushed up in contact with the back of the disks. Each 

 has two turns: this arrangement will be referred to as the disk- 

 galvanometer. 



If a known current is sent through the disk-galvanometer, and the 

 geometrical constant be known, the part of the constant depending on 

 the field and needle is determined. 



The current is measured by a sine-galvanometer, placed in another 



