new Hyperbolagra/ph 



139 



The line P Q, instead of being made of thread, is made of a 

 fine wire of steel wound round a roller at 0, and given one 

 complete turn round the roller at P, fixed to the movable 



T-square P Q. It is kept stretched by means of a string 

 and weight, or a spring pulling it away (not shown in the 

 figure). 



This use of rollers and a fine steel wire, or a band, is very 

 useful in all instruments depending on the use of strings. As 

 a substitute for it, a flat band of steel or copper wire may 

 sometimes also be employed. It 

 will of course be noticed that 

 the rollers form a compensation 

 arrangement, for as much as is 

 rolled on to one roller is rolled 

 off the other. It will also not 

 fail to be noticed, that an ellipto- 

 graph could be constructed in 



this manner by the use of a steel wire and rollers ; the rollers 

 being arranged so as to compensate one another, so that when 

 much wire was rolled up on one, less would be rolled up on 

 the others. This principle has hardly been sufficiently used 

 up to now, and is worthy of attention. In many other instru- 

 ments depending on the length of strings, the direction of the 

 forces altering, there is unequal strain ; but in this instrument 

 (as was pointed out by Mr. Boys) there will be no such 

 tendency, as the string is always kept stretched by a constant 

 weight. 



