﻿Sensitive Form of Thomson Galvanometer. 555 



cementing each set of five, in proper position, to a piece of 

 thin tissue-paper, placing two sets face downward in the 

 proper position, and at the proper distance apart, on a 

 glass " flat," laying the glass staff on top of them, and 

 cementing it to each by means of a very small drop of thick 

 shellac. When dry enough to handle, the other two sets 

 were attached to the other side of the staff, opposite the first 

 two, in a similar manner. The mirror was then attached 

 midway between the two members by means of a minute 

 fragment of soft wax (" universal "), which touched the upper 

 edge only of the mirror ; a method more satisfactory than 

 any other I have tried for mounting small thin mirrors with- 

 out distortion. 



This mirror was 2J millim. in diameter and 1 millim. radius 

 of curvature. It had an accurately worked surface (by 

 Brashear) (rendered necessary by the fact that it is used 

 for a photographic record), and had to be therefore quite 

 thick and heavy for its size. The weight of the mirror was 

 about 12 mgs., and the weight of the whole system about 

 40 mgs. It was magnetized and astaticized after being 

 completed by the method described in my last article. 

 The system was suspended by means of a fine quartz fibre, 

 about 40 centim. long, whose tension was negligible. The 

 method of supporting the fibre is in some respects novel, and 

 has proved very successful in eliminating vibration, which, on 

 account of the lightness of the system, at first proved very 

 troublesome. The glass tube, /, of about 1 centim. bore, 

 which carries at its upper end the adjustable head, h, to 

 which the fibre is directly attached, is not directly connected 

 with the galvanometer-case, but is supported by two thick 

 rings of soft rubber, m, n, very slightly compressed between 

 the glass tube and an outer heavy brass tube which is 

 screwed to the top of the galvanometer-case. There is con- 

 sequently no metallic or solid connexion between the fibre- 

 support and the rest of the instrument, and the vibration 

 which is communicated to the latter from the pier is absorbed 

 by the rubber before it reaches the needle. It is also possible 

 with this arrangement to attach the directing magnet to the 

 outer brass tube without prejudicing the steadiness of the 

 image during adjustment. 



The damping of the needle is effected partly by a piece of 

 dragonfly's wing attached to the back of the mirror and 

 partly by four copper rods, which slide into the cores of the 

 coils. 



The coils on each side are connected in series, and the 

 terminals brought up and connected to two copper binding- 

 posts on the top of the case. The two sides could therefore 



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