﻿554 Mr. F. L. 0. Wadsworth on a very 



Each coil was cemented by means of melted shellac into 

 an open brass case, which left the windings on the face of the 

 coil exposed, and enabled them to be brought as close to the 

 needle system as possible. These brass cases screwed into 

 L- shaped supports, which rested on three adjustable screws — 

 a, b, c, Plate XIV. (placed at three corners of the foot-plate of 

 the L), the points of which slide in V-grooves planed in the 

 metal plate which forms the base of the galvanometer- case. 

 They are clamped in any desired position by means of a 

 screw, d, working in a slot in the plate, as shown. This 

 means of support allows the coils to be accurately centred 

 with respect to each other, and, in conjunction with the 

 levelling-screws on the case, to the needle system, indepen- 

 dently of the adjustments of the latter. It also enables the 

 coils to be readily removed whenever it is necessary to get 

 at the needle, and the distance between the coils to be varied 

 to increase or decrease the delicacy, without altering the 

 astaticism of the system by means of a directing magnet. 



The needle system itself is shown in fig. 4 (PI. XIV.). 

 The central staff is about 150 millim. long, drawn from glass 

 tubing, and weighs about 5 mgs. On account of the length 

 and thinness of the staff, special means of drawing it were 

 necessary in order to get a perfectly straight piece. 



A good method is to clamp a selected piece of tubing, 

 about 5-10 millim. in diameter, in a retort-stand so it 

 hangs vertical, and attach to the lower end a 4-5 lb. weight, 

 which rests on some simple form of trap 4 or 5 feet above a 

 box filled loosely with waste or shavings. The tube is heated 

 uniformly by two good Bunsen burners until it begins to 

 soften, then the burners are removed, the trap is immediately 

 sprung, and the weight fails into the box placed to receive it, 

 drawing out a thin tube of glass, the diameter of which will 

 depend on the length of tube which has been softened. From 

 a few fibres thus drawn a piece can be selected which will 

 be satisfactory as regards straightness and lightness. 



The two members of the system are built up each of ten 

 small magnets, five on each side of the staff, the central one 

 about 3 millim. long, and the upper and lower ones each a 

 little less than 2 millim. They are made from the smallest 

 size sewing-needles broken to the required length, but other- 

 wise untreated *. They were attached to the staff by first 



* This material is unsuited for the purpose, being too soft a grade of 

 steel for retaining a high permanent magnetization. Some bars of special 

 magnet steel were ordered, but have not yet been received, and pending 

 their arrival the above material was used as the best available for the 

 purpose. With steel of proper size, quality, and hardness, the magnetic 

 moment could, I am certain, have b^en more than doubled, without any 

 increase in weight. 



