Platinum Thennometry at Kew Observatory. 7 



Resistance Coils. 

 5. The coil resistances are on the binary scale, 



H = G/2 = F/4 = E/8 = D/16 = C/32 = B/64 = A/128 = Aa/256, 



with two extra coils, " Cal " and FI or I, as it will be called here 

 the former used merely in calibrating the bridge wire. The nominal 

 value of H is 5 box units. The unit is very approximately O01 of an 

 ohm, so that the fundamental intervals of all the thermometers except KG 

 are nearly 100 box units. This has the advantage that when the read- 

 ing R in ice is subtracted from the observed reading, one has approxi- 

 mately the temperature in degrees Centigrade on the platinum scale. 



The coil I is approximately 100 box units, and practically all the 

 fundamental interval determinations, except with KG, have been 

 referred to it. 



The coil Aa was added along with the coil Cal in 1897, when the box 

 was altered. It has not been used in any of the work now to be 

 described, and the mean box unit has been based throughout on the 

 eight coils H to A. The coils are of platinum silver. 



The bridge wire is fully 30 cm. long, readings to the right of the 

 centre representing a temperature above, and readings to the left a 

 temperature below that answering to the sum of the coils whose plugs 

 are out. The bridge wire is also of platinum silver, and possesses the 

 same temperature coefficient as the coils. 



The bridge- wire scale is divided to 1 mm., and the vernier reads 

 directly to 0'02 mm., and allows O'Ol mm. to be estimated. A differ- 

 ence of 1 cm. on the bridge-wire reading answers very nearly to 1 box 

 unit, or to a difference of 1 in temperature with all the thermometers 

 except KG. Thus, in referring to differences of bridge-wire readings I 

 shall usually, for brevity, speak of them as differences of temperature. 



Nature of the Observations. 



6. The expediency of using the same coil combinations for all the 

 thermometers except KG was soon recognised. Also a point has been 

 made of using two combinations for each temperature, according to the 

 following scheme : 



K h 



By using the same coil combinations for different thermometers 

 we may at least hope to detect any sensible relative changes, whilst 



