688 PROFESSOR THOMSON ON THE ELECTRO-DYNAMIC QUALITIES OF METALS. 
reducing the conductor so much that the current through it would generate heat 
enough to keep up the required difference of temperatures without any external 
heater. 
59. The new conductor was therefore made of just two slips of sheet iron broad 
enough to admit the whole length of the thermometer-bulbs in the same manner as 
in the conductor previously used ; these slips were bent in the places for the thermo- 
meter-bulbs, but were kept straight and bound close together elsewhere. Gutta- 
percha pipes were cut and cemented upon the iron slips near their ends, so as to lead 
streams of cold water across them. The part of the conductor between these coolers 
was packed round with a large mass of cotton wool, the thermometer-bulbs being 
Fig. 5. 
Steadied in the apertures prepared for them by means of corks, as before (§31), 
The breadth of the conductor was 2 ^ inches, the length between the coolers only 3^ 
inches (instead of 10 inches, as in the iron conductors used previously), so that too 
great a time might not elapse before such a nearly permanent state of temperature as 
depended on the heating effect of the current would be reached. 
60. On the 25th of March, 1854, an experiment was made with this conductor in 
the following manner: — A constant stream of cold water was maintained through 
each of the coolers ; a current from the full nitric acid battery of eight large iron 
cells was sent through the conductor for twelve times four minutes in each direction, 
that is for ninety-six minutes in all, and the thermometers were noted every half- 
minute. 
The actual observations of temperature are required to show the circumstances of 
this experiment, and I therefore give them as follows ; instead of an analytical table, 
such as those by which the results of the preceding experiments were exhibited : — 
