Intelligence and Miscellaneous Articles. 385 



to some extent made use of during the past fifty years. Naturally 

 enough electricity has been utilized in some way or other in the 

 majority of these systems of telethermometry. 



The requirements of the problem seem to be that the device or 

 instrument used at the point, the temperature of which is to be 

 ascertained, shall be of the greatest possible simplicity of construc- 

 tion, involving little or no motion in its parts, so that the liability 

 to " get out of order " shall be reduced to the minimum ; and that 

 at the observing or registering station, the necessary appliances 

 'shall possess a maximum of durability and simplicity — so that a 

 minimum of time and skill will be demanded in making the obser- 

 vations. The whole system must be certain in its indications 

 and correct within a reasonable limit. 



The first of these conditions is apparently sufficiently well satisfied 

 by the thermo-electric-junction, which has probably been more ex- 

 tensively made use of than any other form of electric thermometer. 

 It renders necessary, however, the use of a comparatively delicate 

 galvanometer, and as the electromotive force of a single couple is 

 small (it is difficult to use more than one in general practice) the 

 results are subject to considerable errors arising from unknown or 

 neglected sources of electromotive force. This source of error 

 becomes more important as the range of temperature measured 

 becomes smaller, although it may be almost entirely avoided by 

 care and skill on the part of the operator. The well-known 

 resistance method of Siemens satisfies the same condition very per- 

 fectly, and is certainly capable of giving good results when skilfully 

 applied, at least throughout moderate ranges. 



The desire to possess some form of electric thermometer which 

 might be utilized in the study of certain problems connected with 

 meteorology, especially the observation of soil and earth tempera- 

 ture, and the use of which would not demand greater skill than 

 that of the ordinary meteorological observer, led to the device and 

 construction of the instrument to be described, which may be called 

 a " differential resistance thermometer/' It consists essentially of 

 a mercurial thermometer, not unlike ordinary forms, except that 

 the bulb is greatly enlarged so that the stem may have a diameter 

 of something like a millimetre, and still leave the scale tolerably 

 " open." In one of the instruments already made 1° C. corresponds 

 to about 5 millim. of the scale. Running down through the stem 

 is a fine platinum wire about -08 millim. in diameter. The lower 

 end may be secured in the bulb so that it is kept straight in the 

 bore of the stem, and at the lower end a heavier wire is sealed in 

 the glass, so that metallic contact can be made with this wire both 

 at the upper end and through the mercury at the lower. It is 

 evident that the resistance between these two points will depend 

 largely (but not entirely) on the length of the platinum wire which 

 is above the mercury in the tube, and this will depend on the 

 temperature to which it is exposed. When this temperature rises 

 the resistance is decreased by an amount equal to the difference 

 between that of the platinum wire which disappears and that of 



Phil. Mag. S. 5. Vol. 20. No. 125. Oct. 1885. 2 E 



