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when the primary was thick and the air gap, through which the jais discharged, large, and no little trouble was experienced in this way.
In Fig. 83 is illustrated another form of the bulb constructed. In this case a tube T is sealed to a globe L. The tube contains a coil C, the ends of which pass through two small glass tubes t and t₁, which are sealed to the tube T. Two refractory buttons m and m₁ are mounted on lamp filaments which are fastened to the ends of the wires passing through the glass tubes t and t₁. Generally in bulbs made on this plan the globe L communicated with the tube T. For this purpose the ends of the small tubes t and t₁ were just a trifle heated in the burner, merely to hold the wires, but not to interfere with the communication. The tube T, with the small tubes, wires through the same, and the refractory buttons m and m₁, was first prepared, and then sealed to globe L, whereupon the coil Cwas slipped in and the connections made to its ends. The tube was then packed with insulating powder, jamming the latter as tight as possible up to very nearly the end, then it was closed and only a small hole left through which the remainder of the powder was introduced, and finally the end of the tube was closed. Usually in bulbs constructed as shown in Fig. 33 an aluminium tube a was fastened to the upper end s of each of the tubes t and t₁ in order to protect that end against the heat. The buttons m and m₁ could be brought to any degree of incandescence by passing the discharges of Leyden jars around the coil C. In such bulbs with two buttons a very curious effect is produced by the formation of the shadows of each of the two buttons.
