Juirm of Platinum Resistance-Thevmoinc4er. 



151 



The main point about the thermometer is that a core passes 

 through the centre of the bulb, through which a liquid or gas 

 can be ma Je to flow. It is therefore particularly well adapted 

 for the continuous-flow calorimeter, where the temperature of 

 a flowing liquid or gas is measured. It is quite as easy to 

 construct as the mica-frame type, and it has many advantages 

 over the latter for the particular use for which it was designed, 

 although it would be unsiiited for certain types of temperature 

 measurement. 



To construct the thermometer, a glass tube about 5 mms. 

 in diameter and 25 cms. long is coated by beeswax for a con- 

 siderable length, and a thread is cut in the wax on a machine- 

 lathe. By means of hydrofluoric acid the glass is etched 

 away, and a permanent thread is produced on the tube. When 

 the wax is removed a point of glass is raised on the tube by 

 means of a blowpipe, leaving a margin of about half a centi- 

 metre from the end. The platinum wire is wound on doul^le 

 in the usual way, the point of glass preventing the wire 

 from slipping. The two ends of the coil are fastened down 

 to the glass after winding by points of hot glass, leaving at 

 least 1 cm. fr,ee at the ends. 



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Cored Thermometer. 



These free ends are fused to silk-covered copper wires of 

 about No. 24 gauge in the blowpipe, which serve as con- 

 necting-leads. The copper wires must be longer than the 

 glass tube, and are fastened securely to the tube in two 

 places by binding-thread. The compensating leads of copper 

 wire, connected by a short piece of platinum wire, are bound 

 in the same way to the tube, and the platinum wire caught 

 on a point of hot glass. The length of this platinum con- 

 necting wire will depend on the length of wire in the bulb. 

 Usually it varies from 1 cm. to 3 cms. Besides being a con- 

 venient way of joining the compensating leads together at 

 the bulb, the short bit of wire serves, as suggested by 

 Callendar, the purpose of compensating for the conduction of 

 heat away from the wire by the leads. As soon as the wires 

 are fastened in place the points of glass are made as flat as 

 possible by means of a small blowpipe-flame. A thin glass 

 tube is then selected, which just slides over the bulb and 

 connecting wires. The two tubes are carefully fused together 

 around the end so as not to melt the glass near the wire. At 



