1899.] on Measuring Extreme Temperatures. Ill 



to problems in ventilation, in which the phenomena depend mainly on 

 effusion through relatively large apertures. It would, however, be 

 difficult to adapt to the problem of temperature measurement. It 

 would not be easy to make an aperture which could be continuously 

 varied without changing its shape, and at the same time to measure 

 the change of area with sufficient accuracy, if the area were small 

 enough to prevent appreciable cooling of the thermometer by the 

 current of air flowing through it. There is also the disadvantage 

 that the pressure-difference varies as the square of the current ; so 

 that, if very small currents are used, the effects of viscosity become 

 more important, and the balance ceases to be independent of the 

 current, unless everything is symmetrical and at the same temperature 

 in corresponding parts. 



For these reasons it seems preferable, in applying the Wheatstone- 

 bridge method to air-currents, to employ fine tubes as resistances, 

 and to eliminate the effects of effusion as completely as possible, at 

 least in the resistance-measuring part of the apparatus. With trans- 

 piration resistances the current is directly proportional to the pressure 

 difference, the electrical analogy is much closer, and the theoretical 

 conditions can be very accurately realised. 



The Wheatstone-bridge method of measurement proved to be so 

 exact, and so perfectly adapted to the problem of transpiration ther- 

 mometry, that, after some preliminary experiments, the writer had a 

 very elaborate apparatus constructed, in the year 1893, which was in 

 every detail the exact analogue of an electrical resistance thermometer. 

 The fine wire resistances of the electrical apparatus, in terms of 

 which the change of resistance of the thermometer is measured, are 

 replaced in the transpiration box by a graduated series of fine tubes, 

 which can be short-circuited by means of taps of relatively large bore, 

 corresponding to the plugs of negligible resistance in the electrical 

 resistance box. The galvanometer is replaced by a rheoscope, con- 

 structed after a pattern devised by Joule for a different purpose, 

 which can be made to rival in delicacy the best modern electrical 

 instruments. The pyrometer itself consists of a fine tube of platinum 

 instead of a wire, and is fitted with " compensating leads " to corre- 

 spond with those of the electrical instrument. All the detail of the 

 methods of observation and calibration are faithfully copied from the 

 electrical apparatus, and the results, so far as the measurement of 

 transpiration resistance is concerned, are equally satisfactory. 



Fig. 4 is a diagram of a working model of the transpiration 

 balance, which was exhibited at the lecture. This model has a ver- 

 tical needle for index, and a pivoted mica vane, which is deflected 

 when a current flows through the bridge piece. It is constructed to 

 work on the ordinary lighting-gas pressure, and to give its maximum 

 deflection for a 10 per cent, change of resistance with the gas about 

 half off. With all the taps off, the resistances on either side are equal, 

 and there is no deflection. In the diagram the balance is supposed 

 to have been disturbed by opening one of the taps. The apparatus 



