300 Met! tod of Reading Torsional Angle of Rotating Shaft. 



to cut off any possible stray ultra-violet light but the small 

 electrical effect still persisted. 



In conclusion I should like to express my warmest thanks 

 to Professor J. 8. Townsend, in whose laboratory this research 

 was conducted, and at whose suggestion it was undertaken, 

 for assistance and encouragement always most readily given. 



XXIX. An Optical Method of Reading the Torsional Angle 

 of a Rotating Shaft. By F. J. Jervis-Smith, M.A., Oxon., 

 F.R.Sr 



DURING certain experimental work in which it was 

 necessary that the energy transmitted by a rotating 

 shaft should be known, I devised the following method of 

 reading the angle of twist of the shaft, and found that it 

 gave good results. It is easily applied, and might be used 

 for other similar purposes in the mechanical and physical 

 laboratory. Clear readings can be taken from a pointer, 

 moving over a circular dial which reflects light, while it 

 rotates about a diameter. The optical principle involved is 

 similar to that of the thaumatrope of Dr. Paris, in which on 

 one side of a card the head of a man was painted, and on 

 the other side a hat. When the card was rotated by means 

 of twisted strings attached to the opposite edges of the card 

 the head and hat appeared as one picture. The rationale of 

 the experiment being that the picture of the head is retained 

 by the organs of vision until the hat appears, the two separate 

 impressions thus making one picture. In my apparatus the 

 reflecting dial, made of mirror glass, is fixed so that its 

 plane is parallel with the axis of the shaft, or spiral spring, 

 the angle of twist of which is to be measured. The mirror 

 is perforated in the centre, and through the perforation the 

 pivot which carries the pointer passes. The pointer is de- 

 flected through an angle proportional to that of the twist of 

 the shaft by means of connecting links attached to the shaft 

 and also to a sleeve fixed to the shaft. 



A parallel beam of light is projected on to the mirror dial 

 by means of two plano-convex lenses, as used in a projection 

 lantern. The image of the pointer moving over the divided 

 scale can either be viewed direct by reflexion, or, which 

 is far more convenient, the image can be projected on 

 to a screen by means of two achromatic lenses, and then 

 it may be viewed simultaneously by several observers. This 

 latter method of viewing the pointer is preferable to the 



* Communicated bv the Author. 



