xiii.] MEASUREMENT OF PHENOMENA. 99 



tions of different metals in the thermopile can be increased 

 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 delicacy 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 telecope (p. 287). 

 Such is the delicacy of this method of measuring heat, that 

 Dr. Joule succeeded in making a thermopile which would 

 indicate a difference of o 0< oooi 14 Cent. 1 



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 one 72,oooth part of a second, 

 the point of light would have appeared elongated to an 

 angular extent of one-half degree. In the spark, as drawn 

 directly from a Ley den jar, no elongation was apparent, so 

 that the duration of the spark was immeasurably small ; but 

 when the discharge took place through a bad conductor, 

 the elongation of the spark denoted a sensible duration. 2 

 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, 

 and should serve only as a means of comparison between 

 two or more magnitudes. As a general rule, we should 

 not attempt to make the divisions of the measuring scale 

 exact multiples or submultiples of the unit, but, regarding 

 them as arbitrary marks, should determine their values by 

 comparison with the standard itself. The perpendicular 

 wires in the field of a transit telescope, are fixed at nearly 



1 Philosophical Transactions (1859), vo l- cxlix. p. 94. 



2 Watts' Dictionary of Chemistry, vol. ii. p. 393. 



