XXIV 



ferred on him the honour of knighthood, an honour so variously bestowed 

 that it can only be viewed as a bare recognition of scientific labours 

 which have rarely been equalled in their extent, their variety, and their 

 fruitfulness. 



Charles Wheatstone was not less conspicuous for his sagacity in per- 

 ceiving the practical bearings of ascertained scientific principles, than for 

 his ingenuity in devising suitable mechanical means for their application 

 to useful purposes. He was one of the first, if not the very first, in this 

 country, to appreciate the importance of Ohm's beautiful and simple law 

 of the relation between electromotive force, resistance of conductors, and 

 resulting current ; he thence rightly inferred that in establishing electric 

 intercommunication between two distant stations by means of the deflec- 

 tions of a galvanometer-needle, or the attraction of an iron keeper by an 

 electromagnet, the coil of which is actuated by a current sent from the 

 other station, the resistance of the coil must necessarily be small compared 

 with that of the intervening conductor, and hence that a coil of much 

 larger resistance than was at first contemplated might be beneficially em- 

 ployed : Ohm's law in fact affords the means of determining in any given 

 circuit the quality of coil by which the largest amount of dynamic energy 

 may be imparted to the moving mechanism. 



Very early in his electrical researches Wheatstone perceived the necessity 

 of some more accurate means of measuring resistances and the strength 

 of currents than even the most sensitive tangent-galvanometer presented ; 

 it occurred to him that small differences between two currents might 

 be much more accurately determined by sending them in opposite directions 

 through the coil of a sensitive sine-galvanometer, the needle of which 

 will then be affected only by their difference, than by estimating the small 

 differences of displacement of the needle due to small differences in a 

 single current. Accordingly he devised the well-known " Bridge," which 

 has been, and must ever continue to be, the basis of all accurate quanti- 

 tative determinations in electricity. In this instrument the two por- 

 tions of a divided current are conducted in opposite directions through 

 the galvanometer-coil and again reunited ; and the resistance sought is 

 interposed in one circuit, and " balanced " by known resistance interposed 

 in the other circuit until the needle is brought to zero. The form of 

 this instrument has varied considerably from that originally proposed, 

 but the principle is precisely the same. 



As necessary adjuncts to this instrument, Wheatstone proposed as a 

 unit of resistance one foot in length of soft pure copper wire, weighing 

 100 grains, and constructed a series of " resistance-coils," each coil con- 

 taining some decimal multiple of the proposed unit ; he also devised the 

 " rheostat," consisting of a fine wire wound in the thread of a screw cut 

 on the surface of a cylinder of non-conducting material, any portion of 

 which wire may be put into or taken out of the circuit by merely turning 

 the cylinder on its axis by a handle. But several circumstances rendered 



