96 Insulation of Wires in Platinum Resistance Thermometers. 



formed and carefully arranged within its tube, short- 

 circuiting becomes impossible. 



2. When for a given purpose a short wire offers too small 

 a resistance, BurstalFs plan becomes for highly attenuated 

 wires, imperfect ; this is due to the absence of the necessary 

 stiffness in the wire. The defect is successfully met by 

 inserting a thin slip of mica between the two limbs of the 

 loop, as shown in fig. /3. The upper end of the slip is held 

 between the two stouter pieces of mica to which are fastened 

 the thermometer leads, and the other end is notched for the 

 reception and retention of the lowest part of the loop. As an 

 alternative. the slip may be pierced with a needle near the lower 

 end, the resistance wire being then threaded through the hole, 

 drawn taut, and finally fused to the leads. Loops of any desired 

 length may be mounted and highly insulated in this way. 



3. For a thermometer having a high resistance, the sup- 

 porting insulator consists of two plates of mica each about 

 1 mm. thick, and cut as shown in fig. y. The wide and 

 shorter portions of tl e plates are held between two others 

 which are used for supporting the platinum, gold, or silver 

 leads, and all four firmly bound together with Hue nickel 

 wire. One lead is made to terminate just below the com- 

 pound block ; but the second lead is longer and its lower 

 portion is hammered flat and placed between the two narrow 

 limbs of the central slips of mica, with its end slightly 

 projecting. The resistance wire having been fused to the 

 longest lead, is wound spiral-wise, as shown in fig. S, upon 

 twin slips, and finally fuse'd to the second and shorter lead. 

 It is then ready to be introduced into its protecting tube, 



-1. More recently we have adopted the following convenient 

 and very efficient plan for insulating and supporting highly 

 attenuated wire loops oL : any required length. A glass rod 

 about 1 or 2 mm. in diameter is prepared and a needle-eye 

 formed at one end. Having fused one end of the resistance 

 wire to the lead, the other end is drawn through the eye in 

 the glass rod and brought up and secured to its lead. Holding 

 the leads vertically, the rod will, if of appropriate weight, 

 pull and maintain the loop taut. The wires are now placed 

 within their protecting glass tube, the lower end of which is 

 open and of such dimensions that the rod suspended from the 

 resistance wire can just pass through. Still keeping the 

 thermometer vertical, and applying a suitable blowpipe-flame, 

 the lower end of the tube is closed by fusing it to the glass 

 rod. When cold, the superfluous glass is cut off and the end 

 rounded in the flame. Fig. e shows a completed thermometer- 

 bulb' of this form. 



Daubeny Laboratoiw, 

 Oxford. 



