All the liquids and salts employed were chemically pure; the solids, how, were only commercially pure. Solids with Liquids and Liquids with Liquids in Air. Example of the above table:-Lead is positive to distilled water, and the contact difference of potentials is 0.171 volt. The authors point out that in all these experiments the unknown electromotive forces of certain air contacts are included. From these tables we find we can build up the electromotive forces of some well-known cells. For example, in a Daniell's cell there are four contact differences of potential to consider, and in a Grove's cell five, viz. :— 207. The electromotive force of a thermoelectric circuit is called Thermoelectric force. It is proportional cæt. par. to the number of couples. The thermoelectric force of a single couple is in the majority of cases equal to the product of two factors, one being the difference of temperature of the two junctions, and the other the difference of the thermoelectric heights of the two metals at a temperature midway between those of the junctions. The current through the hot junction is from the lower to the higher metal when their heights are measured at the mean temperature. Our convention as to sign (that is, as to up and down in speaking of thermoelectric height) is the same as that adopted by Prof. Tait, and is opposite to that adopted in the first edition of this work. We have adopted it because it leads to the rule (for the Peltier and Thomson effects) that a current running down generates heat, and a current running up consumes heat. The following table of thermoelectric heights relative to lead can be employed when the mean temperature of the two junctions does not differ much from 19° or 20° C. It is taken from Jenkin's "Electricity and Magnetism," p. 176, where it is described as being compiled from Matthiessen's experiments. We have reversed the signs to suit the above convention, and have multiplied by 100 to reduce from microvolts to C.G.S. units. Thermoelectric Heights at about 20° C. Antimony, pressed wire + 280 Bismuth, pressed com mercial wire,... -9700 + 300 208. The following table is based upon Professor Tait's thermoelectric diagram ("Trans. Roy. Soc., Edin.," vol. xxvii. 1873) joined with the assumption that a Grove's cell has electromotive force 1.97 x 108 Alloy, believed to be Platinum Iridium, + 839 at all temperatures. Alloy, Platinum 95; Iridium 5,...... + 622- •55 t The lower limit of temperature for the table is 18° C. for all the metals in the list. The upper limit is 416° C., with the following exceptions :-Cadmium, 258° C.; Zinc, 373° C.; German Silver, 175° C. Ex. 1. Required the electromotive force of a copper-iron couple, the temperatures of the junctions being 0° C. and 100° C. |