DESCRIPTION AND APPLICATION OF A TORSION 

 GALVANOMETER. 



BY WILLIAM RITCHIE, A.M., F.R.S. 



Assoc. Mem. S. A. for Scotland, Rector of the Royal Academy of Tain. 



TN a paper which appeared in the first part of the Philoso- 

 ' phical Transactions for 1830, 1 investigated the elasticity 

 of threads of glass, and applied that property to the construc- 

 tion of a delicate and accurate galvanometer. The instrument 

 then described, though sufficient for most purposes, requires 

 some modification to adapt it to researches of extreme delicacy. 

 The description of the instrument, in this more perfect state, 

 with a few of its numerous applications, will form the subject 

 of this communication. 



For experimental researches in electro-magnetism, it is ex- 

 tremely useful to have constantly at hand a quantity of copper 

 wire, of different degrees of fineness, coated with sealing-wax. 

 The most convenient mode of giving the wire this coating, is 

 the following : Stretch the wire between two supports, heat 

 it gradually, from one end to the other, with an iron bar, or 

 spirit-lamp, and continue rubbing the heated part with a stick 

 of sealing-wax ; the wire will receive a fine coating, sufficient 

 to prevent metallic contact when portions of it are pressed 

 together in the construction of any piece of electro-magnetic 

 apparatus. 



Take the wire thus coated, heat it slightly to prevent the 

 wax cracking, and form it into a rectangular shape, consisting 

 of six, eight, or ten repetitions of the wire, according to the 

 delicacy of the instrument required. The upper side of the 

 rectangle must then have the wires separated into two equal 

 portions, bent round a small cylinder, and then continued 

 straight, so as to leave a circular opening in the middle, about 

 one-third of an inch in diameter. The use of the circular open- 

 ing, in the upper side, is to allow a slender axis, carrying the 

 magnetic needles, to pass through it, in order to increase the 

 power of the instrument, and render the compound needle 



