151 
“ Additional Note on the Galvanometer,” by Dr. J. P. 
Joule, F.R.S., V.P. 
The current-meter has enabled me to find experimentally 
the diminution of deflection consequent on an increase of 
the length of the needle. This was done by comparing the 
square roots of the weighings of the meter with the deflec- 
tions occasioned by a coil 15*2 inches diameter in the centre 
of which needles of 1, 2, and 4 inches length were succes- 
sively placed. I find the fractions representing the loss on 
the tangents of deflection as follow : 
From lin. to 2in. 
Needle. 
From lin. to 4in. 
Needle. 
68° 21' 
. *02239 loss 
*11258 loss 
42° 
. *00993 loss 
*04870 loss 
27° 2' 
.. *00023 gain 
*00254 loss 
19° 1' 
. *00287 gain 
*01646 gain 
Hence the total 
fractions gained or 
lost by the two 
needles are : 
For 2in. Needle. 
For lin. Needle. 
68° 21' 
. *02985 loss 
*12009 loss 
42° 
. *01324 loss 
*05194 loss 
27° 2' 
. *00032 gain 
*00230 loss 
19° 1' 
. *00383 gain 
*01756 gain 
Since my last communication I have made trial of many 
bars and needles, and find that the magnetic length is about 
|-th of the actual in most cases. If therefore instead of l we 
take the actual length of the needle n, the correction to be 
applied to the tangent of deflection will be nearly 
1 
4 
-^4tan 2 0 — 1^-^2 sin 20. 
The fractions of loss 
or gain calculated 
from this formula 
for the deflections experimented with are as follow : 
For 2in. Needle. 
For 4in. Needle. 
68° 21' 
,. *02873 loss 
*11493 loss 
42° 
. *01072 loss 
*04290 loss 
27° 2' 
, *00029 loss 
*00116 loss 
19° V 
. *00406 gain 
*01625 gain 
