Measurement of Small Increments of Temperature. 225 



fitted tightly over the ends of this tube. Inside the ends of 

 the quartz tube were fitted short lengths of invar C C, drilled 

 as shown to admit the leads connected to the heating wire XY. 

 Hard-setting sealing-wax was used to make the joints 

 tight. 



The heating wire XY was of constantan, soldered to thicker 

 copper leads, and connected through a low-reading ammeter A 

 and an adjustable rheostat to a battery. The whole of this 

 wire was carefully insulated from the rest of the apparatus, 

 the spaces between the quartz and the heating wire being- 

 plugged with cotton- wool to prevent convection currents. 



The temperature of the copper was determined by a copper- 

 constantan couple, which had been previously calibrated by 

 an accurate thermometer. A constantan wire, 0, soldered 

 along the length of the copper tube, i. e. along GH, formed 

 the hot junction. The cold junction was kept some distance 

 away in a water-bath protected from air currents. 



The thermo-couple leads were connected directly to a 

 suitable galvanometer, the scale of which was so adjusted 

 that '01° C. difference of temperature between the junctions 

 could be read easily. 



A series of preliminary experiments were performed, and 

 it was found that the rise in temperature of the copper tube 

 folio a ed the ordinary laws for increments up to 3° C, which 

 was greater than required for the experiment. Further, the 

 system maintained that temperature over an interval of time 

 sufficient to allosv the plate A to be readjusted. The heat 

 equivalent of the system was found to be 3 74 calories per 

 degree 0. 



To determine the lateral displacement of the plate A, pro- 

 duced by placing weights in the pan, the method used in the 

 original experiment was repeated. Large bending moments 

 were applied to the quartz rod, and the displacement measured 

 by a micrometer, contact being indicated electrically. 



It was found that 1*8 grams placed one inch along the rod 

 displaced the centre of the rod 10~ 6 inch. 



The copper tube was heated to exactly 1° C. above the cold 

 junction. 15'3 grams were required to restore the note to 

 that emitted by the fork. This gives the coefficient of linear 

 expansion of the copper used as '17 X 10~ 4 per degree C, the 

 actual expansion being 'GSxlO -4 inches. 



Thus the arrangement is capable of detecting a change of 



temperature of 6 g^ 1Q _ 4 =000063° C, or about ^ C.,. 

 Phil. Mag. S. 6. Vol. 43. No. 253. Jan. 1922. Q 



