"And what is a thermopile?" queried Carton.
"For the sake of one who has forgotten his physics," smiled Kennedy, "I may say this is only another illustration of how all science ultimately finds practical application. You probably have forgotten that when two half-rings of dissimilar metals are joined together and one is suddenly heated or chilled, there is produced at the opposite connecting point a feeble current which will flow until the junctures are both at the same temperature. You might call this a thermo-electric thermometer, or a telethermometer, or a microthermometer, or any of a dozen names."
"Yes," I agreed mechanically, only vaguely guessing at what he had in mind.
"The accurate measurement of temperature is still a problem of considerable difficulty," he resumed, adjusting the thermometer. "A heated mass can impart vibratory motion to the ether which fills space, and the wave-motions of ether are able to reproduce in other bodies motions similar to those by which they are caused. At this end of the line I merely measure the electromotive force developed by the difference in temperature of two similar thermo-electric junctions, opposed. We call those junctions in a thermopile 'couples,' and by getting the recording instruments sensitive enough, we can measure one one-thousandth of a degree.
"Becquerel was the first, I believe, to use this property. But the machine which you see here was one recently invented for registering the temperature of sea water so as to detect the approach of an iceberg.
