A new thermopile for the measurement of nerve heat-production

Abstract

The thermopile previously described by Downing, Gerard and Hill (1) for the measurement of the heat-production of nerve had four chief disadvantages : ( a ) it gave a relatively small E. M. F. per 1° C., only about 1800 microvolts altogether, about 4½ per couple; in another of these thermopiles, with a smaller number of couples, 1° C. evoked about 9 microvolts per couple. ( b ) This E. M. F. fell off rather quickly, reaching zero within 45 seconds, owing to rapid heat-conduction, ( c ) The temperature of the thermopile took a very long time to settle down, after placing the nerves upon it, viz., about 4 or 5 hours, ( d ) It was very difficult to construct. Of these, ( a ) was due partly to the small length of each couple exposed to the nerves, and partly to the fact that the wire was in contact on its other side with a relatively large mass of insulating material in the wedge on which it was wound; ( b ) was due partly to the neigh-bourhood of this mass of insulating material, which rapidly cooled the wires and nerve, and partly to the conduction of heat away along the wires to the cold junctions which rather soon warmed up; ( c ) was due to the use of a large mass of insulator, which—being a poor heat conductor—allowed only a very slow equalisation of the temperature differences inevitably set up in the initial handling required in placing the nerve in position; ( d ) was chiefly due to the difficulty of fitting the 3 (or 4) wedges exactly together, without gaps, and of joining their very line wires in series. A first attempt to improve the thermopile in these respects was made as follows. To avoid contact of the wires, in the neighbourhood of the junctions, with a mass of insulating material, the wedges on which the wire was wound were milled out along their narrow ends, as shown at ( a ) in the accompanying figure. The wires passed across the gap, the junctions between silver and constantan being in the middle. In this way about 1½ mm. of wire, i. e., ¾ mm. on either side of the junction, was exposed to nerve on one side without being in contact with anything but air on the other. The wedges were of amber, which will stand baking, and the wires were painted with an alcoholic solution of an artificial resin (“ Elo ”) and baked, the process being repeated several times, until a sufficient layer of insulating varnish had been deposited. The use of this varnish has been described already (2, p. 127). It forms a thin, delicate and almost glass-hard sheet of wires and insulator. After mounting the wedges and finishing the carrier a very thin layer of shellac was painted over the (“Elo”) and any small gaps at the edges were filled up with shellac/working under a Zeiss stereoscopic microscope. Then a mixture of paraffin wax and beeswax was spread very gently and thinly over the surface, and melted on with a minute gas flame. This as described in (2), p. 129, renders the surface waterproof. The ends of the grooves in the wedges were then blocked by the paraffin wax, so that water should not get in, the space between the solid wedge and the wire sheet being thereby rendered air-tight. The mass of insulator employed on the carriage of the thermopile was cut down as much as possible, so as to ensure a more rapid equalisation of temperature.