THE EXACT MEASUREMENT OF PHENOMENA. 349 



so that the tension of the electric current derived from 

 the same intensity of radiation is multiplied ; the effect of 

 the current upon the magnetic needle can be multiplied 

 within certain bounds, by passing the current many times 

 round it in a coil ; the excursions of the needle can be 

 increased by rendering it astatic and increasing the deli- 

 cacy of its suspension ; lastly, the angular divergence can 

 be observed, with any required accuracy, by the use of an 

 attached mirror and distant scale viewed through a tele- 

 scope (p. 234). Such is the delicacy of this method of 

 measuring heat, that Dr. Joule succeeded in making a 



thermopile which would indicate a difference of l part 



of a degree centigrade m . 



A striking case of indirect measurement is furnished by 

 the revolving mirror of Wheatstone and Foucault, whereby 

 a minute interval of time is estimated in the form of an 

 angular deviation. Wheatstone viewed an electric spark 

 in a mirror rotating so rapidly, that if the duration of the 



spark had been more than - - of a second, the point of 



72,000 



light would have appeared elongated to an angular extent 

 of one-half degree. In the spark, as drawn directly from a 

 Leyden jar, no elongation was apparent, so that the dura- 

 tion of the spark was immeasurably small ; but when the 

 discharge took place through a bad conductor, the elonga- 

 tion of the spark denoted a sensible duration 11 . In the 

 hands of Foucault the rotating mirror gave a measure 

 of the time occupied by light in passing through a few 

 metres of space. 



Comparative Use of Measuring Instruments. 

 In almost every case a measuring instrument serves, 



m 'Philosophical Transactions' (1859), vol. cxlix. p. 94. 

 n "Watts' ' Dictionary of Chemistry/ vol. ii. p. 393. 



