Comparison of Platinum Thermometers. 4:25 



4. Shunt method of Comparison. — In order to get over this 

 difficulty and secure simultaneous readings, the following 

 method was adopted. The two pyrometers P and Pj were 

 connected in series with the resistance-box B, through a 

 mercuiy-cup switch, so that by rocking the top of the switch 

 they could be short-circuited one after the other, by means of 

 thick copper wires attached to the rocker. Connected with 

 the same switch there was an arrangement such that when P x 

 was short-circuited and P was to be measured, the box B was 

 shunted by means of a mercury connexion. The shunt-box 

 S was then adjusted so that the deflexion of the galvanometer 

 for P x with B unshunted was exactly the same as that of P 

 with the shunt S in parallel with B. The shunt S was a 

 standard resistance-box in ohms with an attachment for 

 reading to tenths. As the shunt S was usually large com- 

 pared with B, it was not necessary to know S nearer than 

 1/10 of an ohm, while the change of resistance, due to the 

 change of temperature of the box, was negligible. Further, 

 an arrangement was made on the same switch for opening 

 and closing the galvanometer circuit, so that the galvanometer 

 was opened first and closed last on rocking the switch. The 

 result was, since, as before stated, the units of the box were 

 so arranged that a deflexion of 20 scale-divisions was the 

 largest necessary, that with a little practice the switch could be 

 rocked and the deflexions of the two pyrometers read almost 

 simultaneously, the needle of the galvanometer not swinging- 

 more than a couple of scale-divisions. The readings could 

 be easily taken within five seconds of each other, and verified 

 again and again without the slightest difficulty, as the 

 temperature slowly changed. The use of the shunt, while it 

 diminished the difficulty of taking observations, somewhat 

 increased the labour of calculation. If (P 2 — Pj) be the 

 difference of resistance of the two pyrometers for any tem- 

 perature, and B the resistance in the box which with the 

 pyrometer P x balances the standard coil M, and if S be the 

 shunt to B for the second pyrometer P 2 , then 



M=P 1 +B = P 2 +BS/(B-fS), 

 whence 



P 2 -P i: =B 2 /(B + S), (3) 



assuming the deflexions of the two pyrometers to be the 

 same. The factor for reducing the units of the shunt-box 

 (ohms) to units of the pyrometer-box was determined from 

 equation (3) by observations taken at 0°, 100°, and 444°* 5 

 (the boiling-point of sulphur) first with the two pyrometers 

 separately using the box B only, and then simultaneously, 

 using the shunt with the second pyrometer. 



