568 DR EDWIN H. BARTON ON THE TEMPERATURE VARIATION 



The source of heat was a gas stove. The temperatures were estimated from the 

 consequent E.M.F. of a thermo-electric couple of platinum and osmium-iridium, one 

 of whose junctions was round the specimen of magnetite and the other in a beaker of 

 water. This couple was first calibrated by placing one junction in melting ice, boiling 

 water, and sulphur vapour * respectively, the other junction being meanwhile at ordi- 

 nary temperatures. 



II. Apparatus. 



5. Electrical Connections. — The electrical connections used are diagrammatically 

 represented in the following figures : — 



Fig. 1. — Primary and Secondary Circuits. 

 Explanation : — 



JIM, Specimen of magnetite. 



P and S, The primary and secondary coils which are 



heated (by gas stove not shown in the figure). 

 P'andS', Compensating primary and secondary coils to 



neutralise the effect of P and S when they 



contain no specimen of rock. 

 B, Battery of Daniell's cells. 



A, Ammeter to indicate the strength of the pri- 



mary current. 



r, Adjustable resistance to keep primary current 



constant. 



K, A Pohl's commutator. 



C, Compensating coil. 



G, A highly-sensitive low-resistance galvanometer. 



Fig. 2. — Thermo-couple and Potentiometer Circuits. 

 Explanation : — 



MM, Specimen of magnetite. 



P, The platinum wire of the thermo-electric couple. 



0, The osmium-iridium wire of the thermo-electric 



couple. 

 HH, The hot junction, formed by tightly twisting together 



the bifurcated ends of the platinum wire with those 



of the osmium-iridium wire. 



C, The cold junction (see also fig. 3). 



6, Highly-sensitive low-resistance galvanometer. 



AXB, A potentiometer read to - 01 cm., about 60 cm. of 



its length being used. 

 R, A constant resistance of 300 ohms. 

 E, A single low-resistance Daniell's cell for producing a 



suitable potential difference between A and B. 



Fig. 3. — Details of Cold Junction. 



Explanation : — 



BB, Beaker containing - 



W, Water. 



SS, Stirrer. 



TT, Thermometer. 



GG, Glass U-tube for insulating the wires. 



C, Copper wire to galvanometer. 



J, Soldered joint of copper wire to P. 



P, The platinum wire which goes to the hot junction. 



There is also another glass U-tube, precisely like the one shown, but containing the 

 junction of the other copper wire with the osmium-iridium wire. This second tube is 

 immersed in the same beaker, close beside the other one, so that both are at the same 

 temperature. 



III. Disturbances and their Elimination. 



6. Disturbances and their Elimination — Kick due to S. — The chief disturbances 

 met with in the course of the determination and the manner of treating them are as 

 follows : — 



* The arrangement adopted for obtaining the sulphur vapour and the temperature assigned to it were those given 

 by Messrs Callander and Griffiths (Proc. Roy. Soc, vol. xlix. No. 296, page 56, Dec. 18, 1890) in the paper "On B 

 Determination of the Boiling Point of Sulphur and a Method of Standardising Platinum Resistance Thermometers by 

 reference to it." 



