HALL. — THERMAL AND ELECTRICAL EFFECTS IN SOFT IRON, 601 



of soda, graphite, and asbestos, with perhaps other ingredients, were 

 used for packing and insulation under the flanges of the main bars 

 and within the cylindrical apertures adjacent. 



To prevent leakage at the ends of the numerous guard-ring bars we 

 depended solely on lead washers squeezed hard between the brass cap 

 nuts c and the walls of the pots ; and to lessen the danger of strain- 

 ing the apparatus and producing rupture of the joints around the main 



Figure 2. 



bars at the tightening of these nuts, we used on or in each guard-ring 

 bar just outside the walls of pots, nuts or iron pins, as Figure 1 shows. 

 Although in most cases the lead washers thus used were entirely satis- 

 factory, we shall see later that through one or two joints leakage finally 

 occurred. 



K and K' are the junctions, 1 cm. apart, of a copper-German-silver 

 thermo-electric couple wound spirally in a groove made in the asphaltum 

 coating of the bar a, as shown in Figure 5 of our former paper, which rep- 

 resents a part of the surface of the bar laid out in a plane. L L', M ]\I', 

 N N', 0', and P P' are similar spirally placed couples. Each such 



