340 
NATURE 
[ Fed. 8, 1883 
For verifying the accuracy of the graduation of the 
potential instruments when performed by either of these 
methods, a standard Daniell’s cell of the form proposed by 
Sir William Thomson at the Southampton meeting of the 
British Association is use 1. It is represented in the annexed 
cut (Fig. 4). It consists of a zinc plate at the bottom of 
the vessel resting in a stratum of saturated zinc sulphate, 
on which has been poured, so gently as to give a clear 
surface of separation, a stratum of half saturated sulphate 
of copper solution, in which is immersed a horizontal 
plate of copper. The copper-sulphate solution is intro- 
duced by means of the glass tube shown in the diagram 
dipping down into the liquid, and terminating in a fine 
point, which is bent into a horizontal direction so as to 
deliver the liquid with as little disturbance as possible. 
This tube is connected by a piece of indiarubber tubing 
with a funnel, by the raising or lowering of which the 
sulphate of copper can be run into or run out of the cell. 
By this means the sulphate of copper is run in when the 
cell is to be used, and at once removed when the cell is 
no longer wanted. 
The electro-motive force of this cell has been determined 
very carefully and found, according to Lord Rayleigh’s 
latest determination of the ohm, to be 1'07 volt at ordinary 
temperatures. The direct application of this cell to the 
galvanometer gives. at once a check on the graduation. 
As the resistance of the galvanometer is always over 
6,000 ohms, there is practically no polarization. 
Fic. 4. 
The method adopted for the graduation of the current 
galvanometer is precisely the same as that first described 
for the potential instrument. The standard galvanometer, 
of which in this case the low resistance coil is used, and 
the current galvanometer to be graduated are joined in 
series witha battery, which with some resistance in circuit 
is sufficient to give a deflection in each of about 45°, when 
the magnetometer of the current instrument is at a con- 
venient position on its platform. The current flowing in 
amperes is given by the standard, and this, of course, is 
the number of amperes which is indicated by the deflec- 
tion of the current instrument. By an obvious calculation 
from the value of H at the current instrument, precisely 
similar to that above described, the number of divisions 
of deflection corresponding to a current of one ampere 
for a field of unit strength is found, and from this the 
series of positions on the platform and their numbers are 
found. 
The value of the field intensity 
the magnetometer when in position has generally been 
determined in the following manner. A battery of about 
30 of Sir William Thomson's Tray Daniell’s is joined 
in series with a resistance of about 7,000 ohms. 
electrodes of a potential galvanometer, on which the 
magnetometer is placed without its magnet, are attached 
given at the needles of 
at two such points in this resistance that the deflection of | 
the needle produced is from 30 to 40 division on the 
The | 
scale. The current through the palvanometer is now 
stopped and the magnet placed in position, and the 
index brought to zero by turning the magnet by means of 
its screw. The electrodes are now placed so as to include 
a resistance which makes the deflection nearly what it was 
in the former case. Let & be the electromotive force of 
the battery, 7 the intensity of the horizontal component 
of the field produced at the needles by the magnet ; 
2, R, the amount of this resistance included between 
the electrodes of the galvanometer in the first and second 
cases respectively ; /, and V, the potential difference in 
volts on the instrument in the same two cases ; D, and D, 
the corresponding deflections, and G the resistance of 
the galvanometer. We have by Ohm’s law 
ERG 
’ V.= GER-P)RF OER GH MAD, 
an 
We = ER,G 
2 = (GSR RIC ERREG = ee 
where 77 is a constant. 
Therefore we have 
D,Rz{(B+R — Ry)(R, + G) +R, G} 
Dz Ry {((B+R— R2)(R,+G) +R, G} 
If the resistance B of the battery be small in compari- 
son with G, or if the galvanometer is sensitive enough to 
allow = to be made sufficiently small by resistance 
added to G, B may be neglected; and it is generally 
possible, by properly choosing &, Aj, 2, to simplify very 
much this formula. The number / thus found, diminished 
by H, is the number of c.g.s. units which measures the 
horizontal intensity of the magnetic field produced at the 
needle by the action of the magnet alone. The value of 
I —H is the number painted on the magnet, and is 
generally about 9 or Io. 
I — H may be determined by means of a current galva- 
nometer very easily by keeping a constant current flowing 
through the instrument, and using without the magnet 
one of the less sensitive positions of the magnetometer, 
and with the magnet one of the more sensitive positions. 
If D, and D, be the deflections and 7, 7, the number of 
divisions of deflections corresponding to one ampere at 
the two respective positions, the value of 7 is at once 
found from the obvious equation 
Dee Ds 
Piglet wien my D, a 
When either instrument has been graduated for a fie} 
of intensity equal to 1 ¢.g.s. unit, and the intensity of the 
field given by the magnet at the needles has been deter- 
mined, the graduation of the instrument is complete. In 
the practical use of the instrument with the magnet in 
position, the number of volts, or the number of amperes 
(according as the instrument used is a potential or a_cur- 
rent galvanometer) corresponding to a deflection of any 
number of divisions is found by the following rule :— 
Multiply the number of divisions in the deflection by 
the number on the magnet increased by the horizontal 
intensity of the earth’s field and divide by the number 
at the division on the platform scale exactly under the 
Front of the magnetometer. 
When the magnet is not used the rule is the same as 
the above, except that the divisor to be used is the value 
of H for the place of the galvanometer. 
For convenience in the ordinary use of either instru- 
ment a position of the magnetometer on its platform, 
| which may not be one of the series described above, is 
determined at which the deflection with the magnet in 
| position, for one volt or one ampere, is one or some other 
convenient number of divisions. For this position lines 
_ 4D, 
1 The mean value of 1 for Great Britain may, wen the magnet is used, 
\ be taken with sufficient accuracy as “17 C.g-S. 
