on a Permanent Klectric Current. 169 
differ many per cent from the mean thickness indicated by the 
weight 
All these sources of error being considered, the discrep- 
-ancies which will be observed in the results to be given, will 
not be surprising. 
A single complete series of observations consisted of ‘the 
following parts: 
Ist. A determination of the extent to which the indicator of 
ence of the magnet and the magnetizing current.—All that it 
one or two mm. and subsequent readings of the Thomson 
galvanometer were, when it was necessary, corrected accord- 
ingly. 
2nd. A determination of the strength of the magnetic field.— 
This was done by withdrawing suddenly from the field a small 
coil consisting of a few turns of wire and observing the effect 
of this action on a delicate galvanometer placed in circuit with 
the coil.* The galyanometer was used with a mirror and scale 
and the readings actually obtained were reduced by the formula 
: ee ; 11 my! 
ee aye ae 
where v is the actual reading and r the distance from the mir- 
ror to the scale. The constant of the galvanometer not being 
known, its sensitiveness, that is the significance of its readings 
in absolute measure, was determined whenever the strength of 
the magnetic field was to be found. This was effected by means 
of an earth inductor placed in circuit with the galvanometer and 
the test coil used with the magnet. he determination of the 
strength of the magnetic field therefore involves two series of 
Std bdeacca one with the earth inductor and one with the test 
coil, 
3d. A determination of the sensitiveness of the Thomson 
galvanometer.—This was done by sending through it a current 
of known strength obtained by shunting the current from a 
Bunsen cell, the main current being measured with a tangent 
galvanometer. 
4th. The main experiment.—The primary current through 
the metal strip measured with the tangent galvanometer just 
spoken of, and the effect of reversing the magnet observed on 
the scale of the Thomson galvanometer. 
5th. Another determination of the sensitiveness of the Thom- 
son galvanometer.—Method as described above. 
* Rowland, “On a Magnetic Proof Plane,” this Journal, vol. x, p. 14, 1875. 
