N A JURE 



[yan. 14, 1 886 



foregoing description it is evident that, once moved, the 

 sector is required to remain in that position imti! moved 

 again ; in fact, it must turn with a certain amount of stiff- 

 ness. A short piece of cord lies in a groove cut in its 

 edge as shown at s ; as the cord is prevented from moving 

 and kept in tension by an india-rubber band, its friction 

 in the 'groove must be overcome when the sector is 

 turned. The smaller of the two grooves on the pulley 

 Q is not visible in the figure, neither is the arrangement 

 shown for lifting the cotton band out of this groove. The 

 instrument being driven is a rotating cylinder for record- 

 ing any vertical movements in the ordinary manner. Five 



groove? of different sizes are cut in the pulley R, and, as 

 there are two grooves in the pulley Q, ten different speeds 

 are possible. The sizes of these pulleys are such that the 

 ten speeds form a geometrical series in which two con- 

 secutive speeds are in the ratio of 100 : 140. 



The cotton band as well as the main driving-cord can 

 be slipped off the driving-pulley without being cut ; it can 

 then at once be removed from the table. From this de- 

 scription it will be seen how the instruments may be 

 driven, as before stated, whilst standing on any part of 

 the table. 



This method of distributing power is convenient for 



laboratories where a large amount of class-work is done, 

 as it removes the necessity of supplying each student 

 with a separate instrument containing clockwork. It is 

 also most useful in original work for driving special 

 pieces of apparatus which may often require more power 

 than can be obtained by clockwork. 



Cotton has been found to be the best material for the 

 cords ; it has the advantage of running almost silently, 

 and is very durable. The pulleys that guide the cord 

 from the shaft to the various rooms have been designed 

 to run a long time without requiring oil, and with very 



little friction. The speed adopted is 10 feet per second ; 

 a small cord running at this speed can easily do all the 

 work that is required. 



A great variety of instruments are driven in the labora- 

 tory at Cambridge ; among others we may mention a 

 turning-lathe, also a small centrifugal machine which 

 runs at about ninety turns per second, requiring a special 

 driving-pulley. 



The whole apparatus, both at Manchester and at Cam- 

 bridge, has been designed and constructed by the Cam- 

 bridge Scientific Instrument Company. 



