PROFESSOR THOMSON ON THE ELECTRO-DYNAMIC QUALITIES OF METALS. 705 
give no deviation, or of the last and first which give the contrary deviations, or of all 
these readings together, according to the nature of the memoranda made by the 
observer, is taken as a determination of the neutral point of the two metals, that is, 
the temperature at which they are thermo-electrically like one metal, or thermo- 
electrically neutral to one another. In the course of one experiment several such 
determinations, both with descending and with ascending mean temperature, are 
made, and if possible also, with first one and then the other junction higher. 
97 . Either in one experiment, or with the same apparatus on successive days, 
determinations are sometimes made with as considerable a variety of differences of 
temperature between the two junctions as is attainable. Sometimes the difference of 
temperatures used is so small as to give very slight indications of electro-motive 
force, even when the mean of the temperatures of the junctions differs widely from 
the neutral point, in which cases, of course, the test is deficient in sensibility. The 
best determinations are generally those derived from observations showing the 
galvanometer at zero, with the widest difference between the temperatures of the 
junctions, to which the thermometers are applicable with trustworthy indications; 
as, for instance, 100° or 150° Cent., which are attainable in the most favourable cases, 
being those in which the neutral point is at about midway between the temperatures 
of freezing and boiling water. The differences between these determinations some- 
times amount to a degree or two, and even to several degrees when zinc was one of 
the metals ; but generally the final mean for the neutral point does not differ by more 
than a degree from any single determination considered as satisfactory at the time it 
was made. 
98. The mutual interchanges of thermo-electric order observed in various specimens 
of zinc, gold and silver, occasioned considerable perplexity, which has only been cleared 
up by observations made subsequently to the communication of this paper. The follow- 
ing determinations were made at different times and by different observers, as noted : — 
Observer. 
Mr. C. A. Smith 
Mr. C. A. Smith 
Mr. C. A. Smith 
Mr. C. A. Smith 
Mr. G. Chapman and 1 
Mr. J. Cranston ... J 
Mr. G. Chapman and \ 
Mr. J. Cranston ... j 
Mr. G. Chapman andl 
Mr. J. Cranston ... J 
Mr. J. Murray 
Mr. G. Chapman and 1 
Mr. J. Cranston ... j 
Mr. G. Chapman and 1 
Mr. J. Cranston ... j 
Mr. G. Chapman and! 
Mr. J. Cranston ... j 
Mr. J. Murray 
Date. 
Sept. 27 , 1854 I 
Aug. 18, 1854 I 
Sept. 8, 1854 | 
Sept. 20, 1854 | 
Jan. 29 , 1856, and Feb. 5. 
Feb. 1856 
Feb. 1856 | 
Aug. 1856 
Feb. 1856 I 
Feb. 1856 | 
Feb. 27 , 1856 | 
Aug. 21, 1856 I 
Metals. Neutral points. 
Pi mean 1 
Gold electrodes J 
Pj mean 1 _ 
Silver electrodes / 
Pf mean 1 , g.g 
Zinc electrodes / 
Silver mean 1 
Zinc specimen (l) electrodes / 
Silver electrodes 1 
Zinc specimen (2) mean ... J 
Silver mean 1 
Zinc (1) electrodes / 
Silver mean ......1 58-18 
Zinc (2) electrodes J 
Silver mean 1 cf-.qr 
Zinc electrodes / 
Gold mean \ 
Zinc electrodes / 
Gold mean 1 69*76 
Zinc electrodes / 
Silver electrodes 1 ^Q.g 
Silver mean T _ 
Gold electrodes J 
