CONVECTION OF HEAT BY AIR CURRENTS. 41 



following parts : — (1) The heated body; (2) arrangements for producing a current of 

 air whose speed could be varied ; (3) an instrument for registering the speed of the air 

 current ; (4) means for determination of the temperature of the body from time to 

 time. 



The body experimented upon was a copper ball, 2 inches in diameter. A circular 

 hole, f inch in diameter and 1^ inch in depth, was bored radially so as to admit a 

 thermo-electric junction. Opposite to this hole a copper hook was screwed into the ball, 

 by which means the ball was suspended. Previous to each experiment the surface of 

 the ball was cleaned, and then carefully blackened by exposing it to the sooty flame of 

 a turpentine lamp. Care was taken to blacken the ball as nearly as possible in the 

 same way for each experiment.* It was then heated in one of Fletcher's circular gas 

 furnaces ; a piece of fine wire gauze being placed between the ball and the flame from 

 the air-gas jet, to prevent the flame either burning or blowing off the lampblack 

 deposited on the surface (see fig. 1 in section and fig. 2 in plan). During the process 

 of heating, the wire suspending the ball was twisted so as to make the ball rotate 

 rapidly about its vertical diameter, in order to prevent any one side of the ball being 

 heated more highly than another. For each experiment, the ball was heated for the 

 same time, viz., fifteen minutes, and when taken out of the gas furnace its temperature 

 was approximately 400° Centigrade. 



The arrangements for producing a steady current of air consisted of one of the 

 Blackman Ventilating Company's fans (fig. 5, E), 32 inches diameter, fitted into a 

 triangular frame, GF, in the side of an air-tight wooden box, A B C D, whose dimensions 

 were 5 feet in length, 6 feet in breadth, and 6 feet in height. The fan had a pulley, H, 

 on its outer side, and by means of a belt passing over this to a shafting driven by a gas 

 engine the fan was made to revolve at a sufficiently high speed, and thereby exhaust 

 the air in the box. Into a circular hole in the box, on the side opposite to that holding 

 the fan, one end of a tinned iron tube, KL, 5 feet long and 6 inches in diameter, was 

 fitted. When the fan revolved air was drawn into the box through the tube. The 

 speed of the fan was the same for all the experiments, but in order to obtain different 

 speeds of air through the tube a movable, slit (fig. 3 longitudinal section, fig. 4 trans- 

 verse section) was placed at the end of the tube where it entered the box ; and by 

 widening or narrowing the slit the speed of the air current could be increased or 

 decreased between the limits of 10 and 1000 metres per minute. 



It was necessary to obtain a current the stream lines in which were parallel to the 

 axis of the tube. That such was obtained was proved by allowing smoke (tobacco 

 smoke or sal-ammoniac fumes) to be drawn into the tube at L when the fan was 

 working, and noticing the direction taken by it in passing through the tube. The 

 necessity for such a precaution was noticed during some preliminary experiments made 

 with a fan of 12 inches diameter fitted to the wider end of a conical tube. The 



* It is very desirable that some coating be found, similar to that of soot or lampblack, which will not readily rub 

 off, or be affected by steam or exposure to a high temperature. 



