66 Mr. A. Stansfield on some Improvements in 



The author considered two methods for avoiding the large 

 deflexion of the sensitive galvanometer and thus rendering its 

 indications more trustworthy : — (1) that of maintaining the 

 a cold junction "of the thermo-couple at some high temperature, 

 such as the boiling-point of sulphur, thus measuring differences 

 between two high temperatures ; and (2) that of balancing 

 the greater part of the E.M.F. of the thermo-couple by a 

 potentiometer method, recording the small remainder by 

 means of a sensitive galvanometer on a moving photographic 

 plate. The latter method was adopted as being much more 

 convenient and generally applicable. 



The New Form of Recording Pyrometer. 



The Potentiometer. — As the ordinary stretched-wire form 

 of potentiometer, with sliding contacts, would have been 

 unsuitable for the purpose, a special instrument was con- 

 structed with plug contacts by means of which any number, 

 from 1 to 99, of equal units of E.M.F. could be inserted in 

 the galvanometer circuit. 



The potentiometer, which is shown at B in rig. 1, consists 

 of four sets of resistance-coils ; a, a' are sets of nine resistances 

 of 2 ohms each, and 6, ¥ of 0'2 ohm each, while c is a set of 

 1000 ohm coils which are placed in the battery circuit. The 

 coils a, b are always in circuit with the galvanometer G and 

 thermo-couple Tc. The battery current flows through the 

 set of resistances c and through parts of the resistances in a 1 

 and 6', and the circuit is completed by the resistances in a 

 and b which lie between the plugs P, P'. 



The terminals of the resistances in a, a' are numbered 

 0, 10, 20, &c. to the left, and in b, V they are numbered 

 0, 1, 2, &c. to the right, so that the readings opposite the 

 two plugs give the number of units of E.M.F. in the gal- 

 vanometer circuit. This arrangement, which involves the use 

 of a larger number of resistance-coils than is usual in 

 potentiometers, was adopted in order that the resistances of the 

 galvanometer circuit and of the battery circuit might remain 

 the same for all positions of the plugs P, P'. The relative 

 resistances of the coils a, b were carefully measured, and a 

 table was calculated giving their values in terms of the mean 

 unit. Changes in the temperature of the potentiometer 

 should not affect the E.M.F. included in the galvanometer 

 circuit, provided that all the coils are at the same temperature. 

 As it was feared that the high-resistance coils c would become 

 heated by the passage of the current to a greater degree 

 than the low-resistance coils, the box B was provided with a 

 zinc lining and the coils were so arranged thev might all be 



