J. Trowbridge—Gaugain's Galvanometer. 385 
Therefore an error in the measurement of R will not’ affect 
the law of the galvanometer so much as one in the measure- 
ment of D. 
In order to obtain an idea of the magnitude of’ errors result- 
ing from changes in the value of D, I arranged a galva- 
nometer so that the coil, consisting in this case of a circle of 
thick brass wire, the resistance of which could be neglected, 
could be moved away from its true position, after the manner 
of Wiedeman’s galvanometer. The value of R was 12 cm. and 
that of D 6 cm. 
Value of D+X Value of 6. 
6 20° 
7 19 
8 18 
8°5 17-5 
16°5 
A variation of 1 cm. produced a change in the deviation of 
the magnetic needle of 1°. (The galvanometer needle was 25 
mm. long and was suspended by a single fiber of silk.) 
The apparatus was also so arranged that the radius of the cir- 
cle of brass could be changed, while D remained constant. It 
was found that the changes in the indications of the needle 
were inappreciable until the increase of the radius was 5 mm. 
when the increase amounted to 1 em.,—a variation of half a de- 
in resistance of the circuit was inappr An error, there- 
fore, of five millimeters in the determination of the ius 
the coil was inappreciable, while t e error in the meas- 
a*.. 3g* . i6-a* 
X=enk} 3% eo or ers 
in which & is a constant depending on the strength of the cur- 
rent, a is distance of point from center of coil, and r is the ra- 
us of the coil. By inspection it will be perceived that a small 
variation in the value of a will affect the value of X more than 
a similar slight variation in the value of r. 
* James Stuart, M. A., Phil. Mag., 46, 1873, p. 231. 
