1884.] New Spring for Electric and other Instruments. 297 



II. " A New Form of Spring for Electric and other Measuring 

 Instruments." By Professors W. E. Ayrton, F.R.S., and 

 John Perry, M.E. Received February 6, 1884. 



In steam and gas engine indicators, the pressure of the fluid on a 

 piston produces a slight shortening of a spiral spring which is mag- 

 nified by a lever, and so the pressure of the steam or gas is recorded. 

 In what are usually known as spring balances there is also occasion- 

 ally a magnification of the elongation of a spiral spring effected by the 

 use of a rack and pinion. Such magnifying arrangements, however, 

 not only introduce inaccuracy by the bad fitting of hinges or of teeth, 

 an inaccuracy which is aggravated by wear, but they increase the cost 

 of measuring instruments and their liability to get out of order. 



And, as an example of the difficulty of using a wheel and pinion 

 for the magnification of an angular motion produced by a small force, 

 we may mention that although we used this plan for a year or more 

 in our electric measuring instruments, and although the wheels and 

 pinions were made by a good watchmaker, still the friction involved 

 in such a plan has induced us to abandon it in favour of the new 

 arrangement which is the subject of this communication. 



The telescopic method employed by Weber, and the spot of light 

 method due to Sir W. Thomson for magnifying the effect of an 

 angular motion are, of course, unequalled for stationary measuring 

 instruments, but for instruments which must be carried about and 

 used quickly without the necessity of adjustment, these most ingenious 

 reflecting methods are quite ivnstiitable. 



With an ordinary cylindric spring, having a small angle between 

 the osculating plane and a plane perpendicular to the axis, as is the 

 case with all spiral springs such as are in practical use, and of which 

 fig. 1 is an example, it is well known that but very little rotation is 

 produced between its ends by the application of an axial force. Con- 

 sequently with such springs, it is only possible to obtain magnification 

 by the employment of a system of levers, or of a rack and pinion. 

 It occurred to us, therefore, to consider whether it would not be 

 possible to make a spiral spring of such a nature that for a com- 

 paratively small axial motion of its ends there should be considerable 

 rotation of one end relatively to the other, and by the employment of 

 which all levers, racks, and pinions could be dispensed with, so that 

 no error could be introduced by wear and tear, or by want of fitting 

 of joints, and further so that the temperature correction should be 

 merely one affecting the rigidity of the material used as a spring, 

 and not a correction such as has to be applied in consequence of the 

 contractions and expansions of the various parts of an ordinary 

 magnifying apparatus. 



