XXV 



Wheatstone's proposed unit unavailable, viz. the difficulty of maintaining 

 uniformity in the size and quality of the wire, the variation of resistance 

 produced by auy molecular disturbance, such as chauge of temperature, 

 stretching or beudiug, and the instability of resistance, even when the 

 wire is protected from mechanical disturbance ; and the British Associa- 

 tion unit is now generally adopted. 



Wheatstoue was the first to appreciate the importance of reducing to 

 a minimum the amount of work to be done by the current at the receiv- 

 ing station, by diminishing as far as practicable the mass and therefore 

 the inertia of the moving parts ; this was beautifully exemplified in that 

 marvel of ingenuity the magneto-electric letter-showing telegraph, now 

 generally adopted for private telegraphic communication, in which the 

 wheel attached to the index is as delicate as the scape-wheel of a small 

 Geneva watch. And there are one or two more points of construction in 

 this admirable instrument which, as exemplars of the mind of the inven- 

 tor, should not escape notice. Earaday had discovered that if a soft iron 

 keeper furnished with armatures revolve in front of the poles of a per- 

 manent magnet, instantaneous currents in opposite directions are deve- 

 loped as the revolving keeper approaches to and recedes from the position 

 of maximum induction ; accordingly Wheatstoue placed four fixed arma- 

 tures coiled alternately in opposite directions in front of the poles of a small 

 but powerful permanent magnet, and made the keeper alone to rotate in 

 close proximity to the iron cores of the armatures ; as therefore the 

 extra currents induced by the keeper in receding from one and ajojiroacJi- 

 ing the other pair of armatures coincide in direction and reinforce each 

 other, four equal currents, alternately positive and negative, will be de- 

 veloped by each revolution of the iron keeper ; and these, by causing a 

 pair of magnetic needles, moving on an axis passing through their common 

 centre of gravity, to oscillate between a pair of small electro-magnets, ac- 

 tuate the index of the receiving instrument. 



Another peculiarly happy and simple contrivance in this apparatus 

 must also be mentioned. The letters of the alphabet surround the trans- 

 mitting-dial ; and opposite each, of these is a stud at the end of a radial 

 lever, the depression of which closes a short circuit, which intercepts all 

 currents when the index arrives at the corresponding letter, and the in- 

 dicator of the receiving instrument consequently stops at the same letter, 

 it having been first ascertained that they start in unison. It then became 

 necessary that the depression of any other lever should raise the one pre- 

 viously depressed ; and for this purpose some very complicated arrange- 

 ments of interacting levers were first devised, but the plan adopted is 

 remarkable for its simplicity : a circular endless chain lies in a groove 

 beneath all the levers, in which there is just " slack " enough to allow of 

 the depression of one lever ; consequently the depression of any other pulls 

 up the former, and opens the short circuit previously closed. 



The catalogue of Wheatstone's valuable labours is still far from being 



