218 
MR. RITCHIE ON THE ELASTICITY OF THREADS OF GLASS, 
require. Simple as this may appear, it requires some practice in manipulation 
to do it well ; it will therefore be necessary to draw several threads of different 
lengths and degrees of fineness, and afterwards select those best adapted to 
the instrument to which it is to be applied. 
6. Description and use of the torsion galvanometer. 
Take a fine copper wire and cover it with a thin coating of sealing-wax. 
Roll it about a heated cylinder an inch or two in diameter, ten, twenty, or any 
number of times, according to the delicacy of the instrument required. Press 
together the opposite sides of the circular coil, till they become parallel and 
about an inch or an inch and a half long. Fix the coil in a proper sole, and 
connect the ends of the wires with two small metallic cups for holding a drop 
of mercury. Paste a circular disc of paper, divided into equal parts, horizon- 
tally on the upper half of the coil, and having a black line drawn through its 
centre and in the same direction with the middle of the coil. Fix a small 
magnet, made of a common sewing-needle or piece of steel wire, to the lower 
end of a fine glass thread, whilst the upper end is securely fixed with sealing- 
wax in the centre of a moveable index, as in the common torsion balance. The 
glass thread should be inclosed in a tube of glass, which fits into a disc of thick 
plate glass, covering the upper side of the wooden box containing the coil and 
magnetic needle. 
The whole will be obvious from the simple inspection of 
the annexed vertical section of the instruments, in which W 
is the coil of wire, C C' the cups, T the glass tube containing 
the thread, I the index turning in the centre of a divided circle 
to mark the degrees of torsion, and N S the magnetic needle. 
By means of this instrument we may compare with great 
accuracy the relative quantities of currents of voltaic electri- 
city circulating along the wires of the coil. For this purpose 
place the needle directly above the line drawn on the paper, 
and consequently directly above, and in the direction of the 
wires forming the upper side of the coil. Cause a current of voltaic electricity 
to circulate along the wires, and the needle will of course be deflected. Twist 
the glass thread, by turning the index, till the needle be brought to its former 
position, and note the number of degrees of torsion ; untwist the thread, and 
©t 
x 
[C y. jyss 
