fi T>R. F. HORTON ON THE EFFECTS OF CHANGES OF TEMPERATURE 



were indicated by two delicate mercury thermometers, one of which had its bulb in 

 the cylindrical copper tube close to the wire and at the middle -of its length ; and 

 the other protruded horizontally into the copper box containing the vibrator, and 

 passed a few millimetres below the plate, the bulb being at the middle of the box (see 

 S and T, fig. 1). These thermometers were graduated to "1 C., and readings to 

 '01 C. could readily be made. In order that the readings of the thermometers 

 might not be affected by the varying temperature of the room, the parts of the stems 

 protruding from the jacket were enclosed in glass tubes about an inch in diameter, 

 through which a constant stream of water was kept flowing. The temperature of 

 this stream was ascertained by means of separate thermometers, one in each water- 

 jacket. 



In the observations at laboratory temperatures the heating jacket was connected 

 to a water tap, and a rapid stream of water allowed to flow through it. It would 

 then, in general, take three or four hours for the temperatures, as indicated on the 

 thermometers, to become constant. The upper one, as would be expected, arrived at 

 a steady state long before the lower, but in no case were observations taken until 

 both had attained a constant temperature. The necessity for this precaution is at 

 once seen, for if the temperature of the thermometer is varying, then it is most 

 probably different from that of the plate or wire, as the case may be. Also, if the 

 plate is at a different temperature from the wire, there must be a flow of heat taking 

 place along the wire, which consequently cannot be at a uniform temperature. 



The other temperatures were all obtained by using the vapours of various liquids 

 boiling under atmospheric pressure. The liquids used were ether (35), acetone (55), 

 methylated spirit (75), water (100), and amyl alcohol (126). In order that the 

 composition of the liquid, and therefore its boiling-point, might not vary, a reflux 

 arrangement was used. This is shown in fig. 1. The vapour is driven from the 

 large copper boiler P, through the tube L, into the top of the heating jacket. It 

 passes out at the bottom, and is condensed in the water condenser N, the condensed 

 liquid running back into the boiler. The boiler is open to the atmosphere through 

 the tube O, which is also surrounded by a water-jacket, and serves as a safety tube. 

 The tube L is a 1-inch "composition" tube, and is fitted to the heating jacket by 

 means of a brass union joint, a washer made of thin sheet asbestos being used to 

 make the joint tight. The tube was well wrapped up in cotton wool, this being 

 covered externally with a cylinder of asbestos cardboard as was also the whole of 

 the cotton wool round the heating jacket in order to prevent it from taking fire in 

 .case of an accident. 



In the first form of the apparatus, the wire alone was surrounded by the heating 

 jacket, and the vibrator hung inside a wooden box, the temperature of which was 

 indicated by a thermometer passing close to the plate. Happening one day to put 

 the thermometer at a further distance from the plate than usual, I was struck by 

 the difference in the temperature indicated. This led me to experiment with several 



