8 DR. F. HORTON ON THE EFFECTS OF CHANGES OF TEMPERATURE 



the wire experimented on was 110 centims. long. On starting afresh at the 

 Cavendish Lalwratory where most of the work described in this paper was carried 

 out this size was reduced to 58 centims., the smaller heating jacket thereby 

 required l>eing more convenient, in that it comes to a steady state more quickly and 

 is more c.isilv maintained at a constant temperature. 



Corrections to the Thermometer Readings. 



The thermometers used were four in number, two ranging from to 100 C., and 

 two from 100 to 200 0., all graduated to '1, These were standardised at the 

 National Physical Laboratory, the tests being carried out with the thermometers 

 under exactly the same conditions as obtained in these experiments, viz., in a 

 horizontal position, and with nine divisions in the hot bath, and the rest of the stem 

 surrounded by a water jacket at a known temperature. From the figures supplied 

 by the National Physical Laboratory, curves of corrections were drawn, from which 

 the corrections to the thermometer readings at any temperature could be read off 

 with an accuracy of '01 C. A small correction had to be applied on account of the 

 difference in the temperature of the water-jacket in these experiments and when the 

 thermometer was standardised. This was found by calculation, taking the coefficient 

 of apparent expansion of mercury in glass as '00016. The correction on this account 

 was never greater than '04 C. 



The method of heating with the vapours of boiling liquids worked extremely well. 

 The jacket would take from 3 to 4 hours to arrive at a constant temperature, and 

 then neither thermometer would vary by '05 C. even in the worst cases, and it often 

 happened that both thermometers remained absolutely steady for hours. It is a 

 curious fact that the corrected readings of the two thermometers were rarely the 

 same. The difference was generally less than '1 C., but sometimes the upper and 

 sometimes the lower one would indicate the higher value. In calculating the results, 

 the temperature of the wire was always assumed to be that of the upper 

 thermometer, while the lower thermometer was taken as indicating the temperature 

 of the vibrator. 



Timing the Vilrrations by a Method of Coincidences. 



The method used consists in observing the reflections of a vertical flash occurring 

 once a second, in two mirrors, one of which is fixed in position, and the other is 

 attached to the vibrating plate and vibrates with it, swinging just above the fixed 

 mirror and l>eiiii, r parallel to it when at rest. The reflections are observed by a 

 telescoj>e, in the field of view of which, in general, two flashes are seen, one always 

 occurring in the same position, and the other appearing in different pails of the Held, 

 according t<> the position of the moving mirror at the instant the flash occurs. If a 



