The imperfections of the thermopile, with which he began his work, led him, about 1880, to the invention of the bolometer, an instrument of extraordinary delicacy, which in its most refined form is believed to be capable of detecting a change of temperature amounting to less than one-hundred-millionth of a degree Centigrade.
Rosse's determinations, like those of his predecessors, were made with the thermopile.
The effect is most easily shown by connecting a voltaic cell to a thermopile for a short interval, then quickly (by means of a suitable key, such as a Pohl commutator with the cross connectors removed) disconnecting the pile from the cell and connecting it to a galvanometer, which will indicate a current in the reverse direction through the pile, and approximately proportional to the original current in intensity, provided that the other conditions of the experiment are constant.
A thermopile is a number of dissimilar junction thermocouples connected in series to increase the voltage for very small heat rise.
At lower power levels a vacuum thermopile and a pyroelectric detector, both with windows and higher sensitivity, maintain the relative response scale.