AS DETERMINED BY FIVE PLATINUM-RESISTANCE THERMOMETERS. 
237 
re-standardise a spare thermometer which was kept in the observing room for general 
purposes. 
This examination led to the discovery of discrepancies in the readings of the 
apparatus which troubled me for a long time, and which necessitated a large number 
of experiments extending at intervals over the greater part of a year before they 
were traced to their sources and eliminated. 
In this part of the work I have to acknowledge the very generous help and advice 
of Mr. E. H. Griffiths, F.IiS., who was kind enough to come to Oxford on more 
than one occasion to place his experience at our disposal, and who. at one stage of 
the investigation, took the resistance box and spare thermometer to Cambridge to 
subject them to a prolonged examination in his own laboratory. 
These discrepancies, though serious in view of the accuracy which we had reason 
to expect from the apparatus, were still small quantities confined within one or two 
tenths of a centigrade degree. They were, for the most part, traced eventually to 
uncertainties in the contacts at the switchboard, and a want of perfect insulation in 
the older leads. These consisted of four india-rubber covered wires which, in the 
underground portion, passed through leaden pipes, but within the observing room 
were without the leaden covering. It was found that these were very susceptible to 
damp, and that the insulation fell away very rapidly when there was much moisture 
in the air, thus giving rise to very puzzling and troublesome discrepancies. 
In September, 1898, the switchboard was improved and new composition cable 
leads substituted, which extended without interruption from the thermometers right 
up to the switchboard. Since these changes were effected the discrepancies have 
ceased to appear, except on one occasion (viz., October 27, 1899), when it was found 
that the short flexible lead from the switchboard to the resistance box was 
thoroughly damp. On lighting a fire in the observing room to dry the covering of 
this lead, the irregularities disappeared. 
Since that date up till the end of March of this year (1900) I have kept a gas 
light burning continuously in the room, to prevent the deposition of moisture, and 
have experienced no further trouble of the sort. 
The resistance box is in its general design similar to that described by 
Mr. Griffiths, # but simplified to suit the particular class of work for which it 
was intended. 
It is provided with three principal coils A, B, C, whose nominal values are 
20, 40 and 80 box units respectively, a box unit being about O'Ol ohm. There are 
two additional coils, one for the calibration of the bridge wire, and another, which 
we have called the “ concealed coil,”' whose value is about 240 box units, which was 
inserted for convenience to balance approximately the resistance of the thermometers 
at 0° C. when the coil A was also in the circuit, so that the reading of the bridge 
wire under these circumstances might be as nearly zero as possible. 
* ‘ Nature,’November 14, 1895. 
