6 HEAT. 



standard, but varies with the variation of gravity. At the equator, 

 760 mm. of mercury would only imply a pressure equal to something 

 less than 758 mm. in England, and water would boil therefore at 

 slightly above 99'9. If great accuracy is required, the standard 

 pressure is taken as 760 mm. of mercury in lat. 45. The same 

 barometric height at Greenwich corresponds to a pressure greater by 

 about 56 in 100,000, which will alter the boiling-point about '016 C. 



Calibration. If the thermometer is to be used as a standard, it must 

 be calibrated, i.e. the variations in the capillary bore must be deter- 

 mined and allowed for. There are various modes of effecting this,* the 

 simplest and quickest being as follows : 



The mercury in the tube is detached near the neck of the bulb, 

 either by warming the tube in a very fine gas flame at the point where 

 it is to be detached, or by manipulation of the trace of air still remain- 

 ing, and which collects in the vacuum left in the bulb if the thermometer 

 is inverted and the mercury is sent down to the end of the tube. The de- 

 tached thread is then run down into an enlargement of the bore provided 

 for the purpose at the end farthest from the bulb, leaving the bore clear. 



A short thread, say about 20 mm. in length, is then detached from 



A B C D . 



FiQ. 4. Calibration of a Thermometer; AB, BC,HK,= equal 

 volumes of the bore. 



the mercury still remaining in the bulb and is measured at various 

 points along the tube by a travelling microscope, or by the dividing 

 engine.f Let us suppose, for example, that the length AK, Fig. 4, 

 along the tube, represents 100, and that we wish to know how to 

 adjust the intervening divisions to allow for variations of bore. Let us 

 bring one end of the thread to A, the other end being at B. Measure 

 AB. Now pass the thread along by gently tapping the end farthest 

 from the bulb till the thread occupies the position BC. Measure BC. 

 Now bring the thread to CD. Measure CD ; and so on. Let us suppose, 

 for simplicity, that the last position of the thread is HK, the end of the 

 thread exactly falling at K, and suppose that there are in all 25 lengths 

 of the thread between A and K. Then the lengths AB, BC, CD HK 

 represent 25 equal volumes, and each of them must contain 4. To 

 graduate to 100, we must divide each of the lengths AB, BC, &c., into 

 four equal parts. At each of the points B, C, &c., there is, therefore, a 

 sudden though small change in the length of the degrees. Hence the 

 method is only applicable when the variation in the bore is practically 

 negligible through one thread-length. For still greater accuracy, the 

 tube is previously graduated to equal lengths, and the correction to be 

 applied at each point is determined by graphic methods. The mode of 

 employing the dividing engine for graduation is described in Stewart 

 and Gee's Practical Physics, p. 24. 



* Report of Committee on Methods Employed in the Calibration of Mercurial Thermo- 

 meters, British Association, 1882. 



t See Stewart and Gee, Practical Physics, vol. i. p. 16. An excellent instru- 

 ment of simple construction is described by Brown in Phil. Mag., vol. xiv. 1882, p. 67. 



