Section 5. Construction of the Bomb. 17 



The lower electrode is made of two steel parts, as shown in fig. 5, the 

 horizontal part C being- afterwards inclosed in a platinum box, which is 

 made as follows : The top A of this box, is made by forcing a circular 

 disk of pure sheet platinum (about 0.25 mm. thick) through a brass die 

 by means of a brass punch. It is better, since it strains the platinum less, 

 to interrupt this operation at least twice, annealing the metal each time. 

 This box should be made to fit so tightly over C that it has to be forced on. 

 In the same way a tight-fitting bottom B is made for this box. A hole 

 is drilled in the center of this just large enough to permit the passage of 

 the steel rod through it. It is then forced on over A. It then remains 

 only to solder B to A with pure gold. This is easily accomplished 

 by putting several pieces of gold on the crack D and directing a hot flame 

 from the blast lamp downwards upon the box. This flame must not be 

 too small, since the whole of the soldering must be done at once and as 

 quickly as possible. Doing it a piece at a time involves keeping the steel 

 rod hot for a longer time, and consequently oxidizing it more ; and worse 

 than this, the gold gets inside and alloys with the iron, bringing the latter 

 eventually to the surface. Before soldering it is better to cover the steel 

 rod below the box with pieces of asbestos, binding them on tightly by 

 means of a wire, so as to diminish the oxidation. 



Before adopting quartz crystal as the insulator various other substances 

 were tried. Mica was tried first of all, using both of the methods sub- 

 sequently employed by Knipp* in his work on surface tension. Our 

 experience agreed with his that it is impossible to secure an absolutely 

 tight joint with mica because of the formation of radial cracks. Nor 

 was it an ideal substance chemically. Carnelian, flint, and agate were next 

 tried because of their known toughness. It was with the last-named sub- 

 stance that we developed the method finally employed for making an abso- 

 lutely tight joint ; the substance itself, however, proved to be chemically 

 unsuitable, since the hydrated silica which it contains dissolves readily in 

 the hot water. The method which we finally employed for securing a 

 tight joint put very little strain on the agate, so that there was no longer 

 any reason for avoiding a substance because of its brittleness. Quartz 

 crystal was then the natural substance to try. Japanese quartz, however, 

 proved a failure, owing to included water or carbon dioxide, which caused 

 it to crack upon heating ; but the Arkansas quartz which we next tried was 

 not affected by heat and has proved to be very satisfactory. Since the 

 thermal coefficient of expansion is so different in the directions parallel 

 to and perpendicular to the main axis of the crystal, the axis of the cup 

 was made parallel to the main axis of the crystal. 



*Phys. Rev., 11, 129-154 (1900). 



