528 Lord Kelvin [May 21, 



electroscope or electrometer. Kohlrausch, in 1851,* devised an appa- 

 ratus for carrying out this kind of investigation systematically, and 

 with a good approach to accuracy, by aid of a Dellman's electrometer 

 and a Daniell's cell, as more definite and constant than a zinc-water- 

 copper cell. This method of Kohlrausch's for measuring the Volta 

 electromotive forces between dry metals, " has been employed with 

 modifications by Hankel, by Gerland, by Clifton, by Ayrton and 

 Perry, by von Zahn, and by most other experimenters on the subject."t 

 About thirty-seven years ago, in repetitions of Volta'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 diagram- 

 matically in Fig. 7, and in perspective in Fig. 8. The two discs 

 are protected against disturbing influences by a metal sheath. The 

 lower disc is permanently insulated in a fixed position, and is kept 

 connected with the insulated pair of quadrants of a quadrant electro- 

 meter. The upper disc is supported by a metal stem passing through 

 a collar in the top of the sheatb, so that it is kept always parallel to 

 the lower disc 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 milli- 

 metre above the upper face of the lower disc. A portion of the wire 

 connecting the lower plate to the insulated quadrants of the electro- 

 meter 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 contacts should be between clean 

 and dry polished metals, because if the last connection on breaking 

 contact is through semi-moist dust, or oxide, or " dirt " (defined by 

 Lord Palmerston to be matter in a wrong place), or if it is anything 

 other than metallic, vitiating disturbance is produced. 



§ 10. To make an experiment, first test the insulation with the 

 upper plate held up in its highest position, and after that with it let 

 down to its lowest position, in each case proceeding thus : Holding 

 by hand the wire connected to the slider, run the slider to zero, make 

 contact at P, observe on the screen the position of the spot of light 

 from the electrometer mirror for the metallic zero, and then run the 

 slider slowly to the top of its scale and break contact ; the spot of 

 light should remain steady, or at all events should not lose more than 

 a very small percentage of its distance from metallic zero, in half a 



* * Poggendorff Annalen,' vols. Ixxv. p. 88 ; Ixxxii. pp. 1 and 45 ; and Ixxxviii. 

 p. 465, 185] and 1853. 



t Prof. O. J. Lodge, * On the Seat of the Electromotive Forces in the Voltaic 

 Cell,' Brit. Ass. Report, 1884, pp. 464-529. 



