which would otherwise tend to break the glass and copper away from each other. Successful disc seals have been made with copper up to 1-10″ thick. There is, of course, a certain maximum thickness that can be used for a seal of a given diameter and it is preferable to keep weel below this limit.
The seals shown in Fig. 4 close the ends of glass tubes to the other ends of which are sealed pilot lamps for the purpose of testing the vacuum. Tubes sealed in this way have been kept a number of years without any impairment of the vacuum.
Fig. 5
The third type of seal and the most important in connection with the present problem is the tube shown in Fig. 5. This furnishes the means of joining metal and glass tubes end to end and is used in the water-cooled tube to attach the anode to the glass cylinder which
