AS DETERMINED BY FIVE PLATINUM-RESISTANCE THERMOMETERS. 
239 
The galvanometer microscope is placed on a window ledge to the right of the 
resistance box, in such a position that the observer can manipulate the commutator 
for reversing the direction of the current without removing his eye from the eye¬ 
piece. 
The general arrangement of the apparatus is shown in fig. 1. To the right is 
the galvanometer and microscope ; underneath in front is the commutator, and 
behind it the contact key. On the extreme left is the switchboard, and in the 
corner of the room is seen a small electric motor for stirring the oil in which the 
resistance coils are immersed. 
In the standardisation of the apparatus the method described by Mr. Griffiths in 
‘ Nature,’ of November 14, 1895, was in the main followed. The temperature coeffi¬ 
cient was determined by Mr. Griffiths when the apparatus was under examination 
in his own laboratory at Cambridge. Two separate determinations made in 1898 
(July 27 and August 8) gave the following results :— 
Date. Range of Temp. Temp. Coeff. 
July 27 . . 9°T8 0-000242 
August 8 . . 12-51 0-000240 
In the reduction of the observations, the value 0"00024 has been adopted. The 
accuracy of this value has been borne out by subsequent observations in several 
different ways. Thus, for example, the invariable steadiness in the changes of No. 5, 
whatever might be the temperature of the box, indicated a high degree of accuracy 
in the adopted value of the constant. 
For the determination of the coil values and the unit of the bridge wire scale, the 
following observations were made at the Iladcliffe Observatory :— 
C — B - A = 20-051 
20-046 
20-043 
B — A = 19-851 
19-849 
19-848 
19-853 
19-847 
A = 19-603 
•600 
•601 
•602 
Means . . 20"047 
19-850 19-601 
From these we obtain, as in Mr. Griffiths’ paper referred to above, 
C = 80U58 t 
B = 39"979 >meau box units. 
A = 19-863 I 
and one scale division of the bridge wire is equal to 
1"0134 mean box units. 
We thus get the following table giving the correction for the particular arrange- 
