50 Mr. H. H. Poole on the Thermal Conductivity 



errors being introduced into the readings of the very small 

 P.D/s of the couples by leakage from the furnace circuil 

 isalmosl entirely obviated. The possibility of a leak from 

 the central wire causing an error was tested, and it was 

 found that switching off the current through the central wire 

 produced no effect on the reading- of the couples until 

 sufficient time had elapsed for the temperature change to 

 reach the couple, so that evidently no leakage of current 

 occurred. 



The current through the furnace is entirely controlled by 

 means of a choking-coil, consisting of a solenoid containing 

 about 250 turns of insulated copper wire 3 mm. in diameter 

 wound on a glass tube 30 cm. lono- by '2"2 cm. internal 

 diameter. In this fits a bundle of varnished soft-iron wires 

 furnished at one end with a pointer moving over a scale, so 

 that the position of the core can be noted. By moving the 

 core into the coil the current is reduced, and vice versa. By 

 this means an exceedingly long range of continuous control 

 is obtained, and as comparatively little energy is wasted in 

 the choking-coil, it does not overheat seriously. When the 

 furnace is being heated up in the morning the choking- 

 may be short-circuited by means of a suitable -witch. A 

 Siemens dynamometer was converted into a wattmeter by 

 making new windings, and used to measure tic energy 

 supplied to the furnace. As the instrument wa>* only roughly 

 calibrated its readings are only relative, but arc useful in 

 adjusting the supply of energy to the furnace so a- t.» obtain 

 a constant temperature as quickly as possible. 



The wires leading from the couples arc connected to a 

 mercury switch so that either couple may he connected to the 

 potentiometer. This switch, which is designed to eliminate 

 irregular thermal effects and somevh.it resembles one for a 

 similar purpose described by Lees, is illustrated, partly in 

 section, in fie;. 3. A is an ordinary bayonet lamp-holder 



7 O I 



connected to electric light twin flex in the ordinary wav 

 except that in addition to the copper conductors a -tout 

 platinum wire is tixed in each screw-contact. I >n the lamp- 

 holder are screwed as many shade-holder rings a- will tit 

 to increase the mass of metal surrounding the platinum- 

 copper contacts enclosed, and so ensure their equality el" 

 temperature. The open end of the bayonet-holder is imbedded 

 in a slightly tapered cylinder of paraffin-wax B in the top 

 of which i* :i cylindrical cavity ( '. In the centre of this is 

 fixed a thin glass tube D round whicb are fixed ben quill 

 glass fcul es E. The platinum wires leading from the bayonet- 

 holder, and BO I rem I he potentiometer, lead into the central 



