422 



Dr. S. Bid well. On the Changes of 



[Apr. 11, 



fig. 1, which, for the sake of clearness, is not drawn to scale. AB is a 

 compound rod made by soldering together end to end five straight 

 wires in the order (1) copper, (2) iron (or other magnetic metal), 

 (3) copper, (4) iron, (5) copper. The length of each iron wire M, N, 

 is 10 cm., and that of the copper wire between them 12 cm. The 

 middle and end portions of the rod are surrounded by three separate 

 brass tubes, in the sides of which are inlet pipes, C, G, H, D, and outlet 

 pipes, E, K, F, for the admission and egress of water and steam. The 







M 



K 

 Fig. 1. 



rod is held axially in the tubes by well-tallowed corks, through which 

 it passes, each of the four copper-iron junctions being exactly opposite 

 an inlet. Cold water enters at C and D and emerges at E and F ; 

 steam from a small boiler is admitted at G and H, escaping at K. It 

 was ascertained experimentally that with the low resistances and 

 small electromotive forces employed water was a good insulator. The 

 iron wire M is magnetised by the coil PP. The field at N due to the 

 coil is less than a hundredth part of that in the interior of the coil, 

 and is regarded as negligible ; but if the apparatus were to be re- 

 constructed, I should, with my present experience, consider it desirable 

 to increase the distance between M and N. The two ends of the 

 compound rod AB were joined to a galvanometer T of low resistance. 



Method II is diagrammatically illustrated in fig. 2. AB consists of 

 a wire or strip of the magnetic metal M soldered between two copper 

 wires ; it passes through two short and wide brass tubes provided with 

 corks at their ends. Inlet and outlet tubes are fitted to the corks, as 

 shown in the diagram, C admitting cold water and G steam. Cold 

 water also flows through I), F, to equalise the temperatures at A 

 and B. The electromotive force of the thermo-couple is opposed by 

 the potentiometer arrangement shown. S is a dry cell having an 

 electromotive force of 1'46 volt. The low resistance r was either 10 

 or 2 ohms. ; the high resistance R ranged from some hundreds to some 

 thousands of ohms in the different experiments. Before an experiment 



