114 Mendenhdll — Differential Resistance Thermometer. 



Art. XVI. — On a Differential Resistance Thermometer ; by 

 T. C. Mendenhall. 



The determination or registration of the temperature at a 

 distant or not easily accessible point, is so extremely desirable 

 that many methods for accomplishing this end have been pro- 

 posed, and 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 telether- 

 mometry. 



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 

 construction involving little or no motion in its parts, so that 

 the liability to " get out of order" shall be reduced to the mini- 

 mum ; 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 observations. The whole system 

 must be certain in its indications and correct within a reason- 

 able limit. 



The first of these conditions is apparently sufficiently well 

 satisfied by the thermo-electric-junction which has probably 

 been more extensively made use of than any other form of elec- 

 tric 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 consider- 

 able errors arising from unknown or neglected sources of elec- 

 tromotive force. This source of error becomes more important 

 •as the range of temperature measured becomes smaller, al- 

 though it mav 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 perfectly, and is 

 certainly capable of giving good results when skillfully 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 con- 

 nected with meteorology, especially the observation of soil and 

 earth temperature, 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 de- 

 scribed which may be called a "differential resistance ther- 

 mometer;" It consists essentially of a mercurial thermometer, 

 not unlike ordinary forms, except that the bulb is greatly en- 

 larged so that the stem may have a diameter of something like 



