96 
PROFESSOR HUGH L. CALLENDAR ON 
(22.) Differential Measurements. 
The only thermometric measurement requiring the highest accuracy in the present 
investigation was the difference of temperature between the inflowing and outflowing 
water at the two ends of the calorimeter. As this difference was obtained by a 
single reading of a pair of differential thermometers, all the minor errors and 
corrections were practically eliminated. Further, as there was no particular 
advantage to be gained by observing the fundamental interval with greater pro¬ 
portionate accuracy than the differential measurement, many special precautions, 
such as measuring the current heating, or keeping the current constant, were 
rendered superfluous. For the same reason it was unnecessary to secure the highest 
possible accuracy in the comparison of the coils of the box (as only a few’ of the 
smallest were used for the differential measurement), and no attempt was made to 
keep the temperature of the box approximately constant (to minimise possible errors 
due to imperfect compensation, or variation of temperature), although such pre¬ 
cautions would naturally be adopted in many other investigations. It was sufficient 
to make sure that the vernier was correctly set and read, and that the sensitiveness 
of the galvanometer was suitable for the differential measurement. 
The galvanometer employed in this investigation was specially made for the work 
by Messrs. Nalder Bros., to my order, in 1896. It was an astatic instrument, 
with four small coils of 5 ohms each. It had a fairly small and light magnet system, 
and a light plane mirror about 4 millims. in diameter, and was fitted with a long 
silk suspension, and a symmetrical system of control magnets above and below. 
The suspension might have been more delicate, and the magnet and mirror system 
lighter, with a smaller moment of inertia, but as the sensitiveness proved to be 
ample for the purpose, no alterations were made. The most important point for our 
work was the astaticism of the magnets. It was quite impossible to work with a 
sensitive non-astatic galvanometer owing to disturbance from the electric railway. 
But the astatic instruments, if properly adjusted, were very little affected. It was 
necessary to keep the time of swing short to secure sufficient rapidity of observation, 
but even with this restriction there was no difficulty in adjusting the galvanometer 
to give a deflection of four or five scale-divisions for a thousandth of a degree with a 
current of about one two-hundredth of an ampere through the thermometers. The 
deflections were observed with a microscope carrying a fine scale and a micrometer 
eye-piece. No attempt was made to adjust the contact to the exact balance point. 
It was merely set to the nearest millimetre division of the bridge-wire, and the 
galvanometer deflection observed on reversing the battery current. The exact 
balance reading could be easily calculated from this by observing the deflection per 
millimetre of the bridge-wire, which remained fairly constant. The mean temperature 
difference was worked out to the ten-thousandth of a degree. It appears probable 
from the observations that the error very rarely amounted to as much as one two- 
thousandth of 1° on a rise of temperature of 8°. 
