£0 Lord Kelvin on 



§ 9. Hitherto we have had no means of measuring the 

 amount of the Volta-eontact electric force between dry metals, 

 except observation of the degrees of deflexion of the gold 

 leaves of an electroscope, or of the spot of light of the quadrant 

 electrometer, consequent upon operations performed upon dif- 

 ferent pairs of metals, with dimensions and distances of motion 

 exactly the same, and comparison of these deflexions with the 

 steady deflexion from the metallic zero given by polished zinc 

 and copper connected conductively with one another by 

 water , and connected metallically with the two electrodes of 

 an electroscope or electrometer. Kohlrausch, in 1851*, 

 devised an apparatus for carrying out this kind of investiga- 

 tion systematically, and with a good approach to accuracy, by 

 aid of a Dellman's electrometer and a DanielFs cell, as more 

 definite and constant than a zinc-water-copper cell. This 

 method of Kohlrausch's for measuring the Yolta electromotive 

 forces between dry metals " has been employed with modi- 

 fications by Hankel, by Gerland, by Clifton, by Ayrton and 

 Perry, by von Zahn, and by most other experimenters on the 

 subject " f . About thirty-seven years ago, in repetitions of 

 Yolta's fundamental experiment proving contact electricity 

 by electroscopic phenomena resulting from change of distance 

 between parallel plates of zinc and copper, I found a null 

 method for measuring electromotive forces due to metallic 

 contact between dissimilar metals, in terms of the electromotive 

 force of a Daniell's cell, which is represented diagrammatically 

 in fig. 7, and in perspective in fig. 8. The two disks are 

 protected against disturbing influences by a metal sheath. 

 The lower disk is permanently insulated in a fixed position, 

 and is kept connected with the insulated pair of quadrants of 

 a quadrant electrometer. The upper disk is supported by a 

 metal stem passing through a collar in the top of the sheath, so 

 that it is kept always parallel to the lower disk and metallically 

 connected to the sheath, while it can be lifted a few centimetres 

 at pleasure from an adjustable lowest position in which its 

 lower face is about half a millimetre or a millimetre above the 

 upper face of the lower disk. A portion of the wire con- 

 necting the lower plate to the insulated quadrants of the 

 electrometer is of polished platinum, and contact between this 

 and a platinum-tipped wire connected to the slider of a 

 potential divider is made and broken at pleasure. For 

 certainty of obtaining good results it is necessary that these 



* Poggendorff's Annalen, vols. lxxv. p. 88 ; lxxxii. pp. 1 and 45 ; and 

 lxxxviii. p. 465, 1851 and 1853. 



t Prof. 0. J. Lodge, " On the Seat of the Electromotive Forces in the 

 Voltaic Cell," Brit. Assoc. Report, 1884, pp. 464-529. 



