278 Dr. J. S. G. Thomas on the Forced Convection oj 



Experimental, 

 Details of the mode of insertion of the heated wires in the 

 flow tube are given in fig. 1. The flow tube was made in 

 two halves. The wires A were of pure platinum supplied 

 by Messrs. Johnson & Matthey, and their length was as 

 nearly as possible equal to the diameter of the flow tube. 

 They were attached to pairs of copper leads B, B of 0'8 mm. 

 diameter as previously described. The following device 

 was finally adopted as the most suitable for maintaining the 

 wires taut during the course of the experiments. The straight 

 copper leads passed through holes affording a nice sliding fit 

 in blocks of ebonite C fixed to the flow tube as shewn. One 



pair of leads passed in similar manner through holes whose 

 respective contours were about three-quarters of the circum- 

 ference of a circle drilled at the edge of brass pieces D 

 carried by supports G, H attached to the ebonite blocks C as 

 shown. Similar holes through which the individual leads 

 passed were provided in the brass pieces D! and D 2 , and 

 these leads were securely clamped by means of the screws E, 

 the bras s pieces being slotted for this purpose. The wires 

 were maintained taut by means of the weak springs F 

 pressing against the brass supports D 2 , which slid freely 

 along the surface of the supports G, H. The tension in the 

 wires was adjusted by means of the screws J. In general, 

 the variation of resistance of the wires due to tension was 

 less than 0*1 per cent, of their resistance. The pair of leads 

 shown in the upper part of the diagram was connected with 

 screw terminals K by means of leads of loosely stranded 

 copper wires. The distance apart of the heated wires could 

 be adjusted to any desired value by the insertions of annular 

 blocks L, accurately drilled to the diameter of the flow tube 



