﻿

88 Messrs. H. T. Tizard and D. R. Pye on the 



the cylinder head. D and E are balance weights to keep 

 the wires taut, and F is a compound pulley which reduces 

 the horizontal motion o£ the wire Gr to about one-half the 

 vertical movement of the piston. At H the wire Gr is 

 divided, and each half wraps round the periphery of a sector 

 attached to the arm K of the indicator. The barrel of the 

 indicator is thus uniformly rotated through a sector about a 

 vertical axis, and the speed of the point of light horizontally 

 across the photographic plate in the indicator camera can 

 be varied between wide limits by alteration of the weights 

 and the number of holes in the loaded piston through which 

 the paraffin is forced. Actually it was found in these 

 experiments that only two speeds were required, of 5*77 and 

 4* 30 cm. per second. These were obtained when the piston 

 carried weights of 40 and 24 lb. respectively, and was 

 pierced by a single hole £ inch diameter. 



Both the speed and its uniformity were measured by in- 

 terrupting the light at known intervals while it traced a 

 straight horizontal line across, the plate. To obtain the 

 uniform interruptions, 60 equally spaced contacts were 

 arranged round the periphery of a disk about 6 inches in 

 diameter. An electric motor was used to rotate the disk 

 through a friction clutch, the speed being kept steady by 

 the operation of a governor, which caused the disengagement 

 of the clutch when the speed tended to increase. The 

 arrangement was very similar to that of a gramophone 

 motor, except that the speed of the latter is kept steady by 

 means of a little brake which is operated by a governor, 

 whereas in our experiments the governor operated the clutch. 

 This apparatus, which was made at the Royal Aircraft 

 Establishment, Farnborough, was lent to us for these experi- 

 ments by kind permission of the Director of Research, Air 

 Ministry. 



To calibrate the falling weight apparatus, the speed of 

 revolution of the disk was first adjusted roughly to one com- 

 plete revolution in 2 seconds. It was then timed repeatedly 

 over twenty complete revolutions by stop-watch ; the mean 

 time for this number of revolutions was found to be 36*5 

 seconds, the variations in successive timings not exceeding 

 •4 second. Hence, since sixty contacts were made each 

 revolution, the time between the beginnings of successive 



36*5 



contacts was -=7* ztt; = 0*0304 second. The 6-volt lamp of 



60 x 20 r 



the indicator was then connected up, through the contacts 



on the disk, to a 12-volt battery ; it was thus greatly 



" overrun " whenever contact was made. The effect of 



