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Proceedings of the Royal Society of Edinburgh. [Sess. 
with loosely packed cotton- wool. These jackets were found to be necessary 
in order to bring the plate to a stationary condition as regards temperature. 
In some cases they were supplied with cold water, and in others with steam. 
The electromagnet used was of the “ ironclad ” type, and was designed 
by Professor Baily and built up in the workshops of the Heriot-Watt College. 
The pole-pieces had a maximum diameter of 20 cm. and were coned 
down to a pole face of 5 cm. diameter. Each pole carried an exciting coil 
1/vVJ 
/aA 
of 880 turns, so that with a current of 20 amperes a magneto-motive force 
of 35,200 ampere-turns was obtained. This was found to produce a field 
strength of about 20,000 C.G.S. units. 
Measuring Arrangements. 
The free ends of the wires of the five thermocouples were soldered 
to stouter copper wires, and these junctions were kept in a Thermos flask, 
by which means their temperature was maintained constant for the length 
of time required for the observations. 
