82 
PROFESSOR HUGH L. CALLENDAR ON 
1 in 300 for the small coils (576 turns each). The insulation between the coils and 
the metal frame-work was in all cases perfect, as the channels were lined with 
paraffined paper. The wire itself proved to be very carefully w T ound, and the 
double silk covering throughout was uninjured. The defects could not he attributed 
to excessive damp, as the winter climate of Montreal is extremely dry, and the 
laboratory was most efficiently heated. Having regard to the perfect condition of 
the wire, it seemed unlikely that absolute security of insulation could be attained 
with silk-covered wire, however carefully wound. 
Owing to the extreme importance of securing perfect insulation, and of being able 
to test the insulation at any time, the small pair of coils were immediately re-wound 
with a double winding of two wires side by side throughout, with paper between 
each layer. They were then boiled in paraffin wax, and the insulation has since 
proved to be practically perfect. At the same time it was decided to re-wind the 
large coils with a double-winding of copper tape, of the same width as the channel, 
insulated with paraffined paper or silk between the layers. But as there was some 
delay in securing the requisite material, the experiment had to be postponed for the 
time. I took advantage of this delay to work out the details of the following 
improvements, which were introduced into the method on resuming the work in 
October, 1897, with Mr. King’s assistance. 
(11.) Duplex Scale Reading. 
For greater accuracy in observing the deflections and measuring the scale distance, 
which is generally one of the weakest points in a deflection method, a duplex system 
of reading was adopted. The apparatus was erected on a suitable pier, which had 
been provided for it in a basement room, with copper fittings and heaters, where the 
temperature was very steady and the ground free from vibration. A plane parallel 
mirror, ^ inch thick, and 2 inches in diameter, silvered on both faces, was fitted to 
the suspended coils. A pair of metre scales, accurately divided on plane milk-glass, 
were mounted with suitable adjustments on a rigid frame of copper tubes at a distance 
of 3 metres apart, 150 centims. east and west of the mirror respectively. A 
circulation of water was maintained through the copper tubes. The distance 
between the scales and the thickness of the mirror could be measured with great 
accuracy. The deflections were observed simultaneously from either side with a pair 
of very perfect reading telescopes of 2 inches aperture, and about 2 feet long. 
These were fitted with filar micrometers, and adjusted so that one turn of the 
micrometer screw w T as very nearly equal to 1 millim. on the scale. The mirror 
and telescopes were specially made by Brashear for this work. The coils and 
suspension w T ere completely enclosed to screen them from draughts, and observations 
were taken through a pair of thin mica windows. With this arrangement, it was 
optically possible to read with certainty to a fiftieth of a millimetre on a steady 
