THE LEYDEN JAE. 621 



rod with a knob, which has been prepared previously, is stnck 

 into the middle of the filings so as to stand upright. The 

 apparatus is then charged by the electrophorns or the machine in 

 the manner given farther on. If no satisfactory result is obtained 

 another jar is tried until one is found which acts well. This is now 

 provided with the proper coatings, after removing the filings and 

 the paper. 



The rod is made of brass wire, 3 or 4 mm thick (for larger jars 

 about 6 mm ), and about one-third longer than the height of the jar. 

 A leaden bullet does not make a very suitable knob, as it is diffi- 

 cult to give it a perfectly smooth surface. It is better to use a 

 brass knob like those mentioned previously (see page 604), of 12 or 

 15 mm diameter, or about 20 mm for very large jars. As these knobs 

 are generally provided with screws soldered to their inside, they 

 should be held by the crucible tongs in the spirit-flame ; the solder 

 melts and the screw will fall out. There is generally sufficient 

 solder left for attaching the metal rod ; the end of it is dipped into 

 soldering liquid and pressed inside the knob, which is held in the 

 flame. If necessary a little soft solder must be added. The lower 

 edge of the knob, which is usually rather rough, should be 

 smoothed with the file ; this is best done after fixing the metal 

 rod, which serves for holding the knob during the operation ; it is 

 somewhat difficult to file the edge smooth while holding the button 

 by itself in the hand, nor can it be clamped between the jaws of a 

 vice without running danger of crushing it out of shape or breaking 

 it. 



Instead of leaving the knob hollow it may be filled with lead. 

 The end of the rod is first covered with a little soft solder ; the knob 

 is then held in the flame with its opening upwards, and small bits 

 of lead are thrown into it until it is full of molten lead ; the rod is 

 now seized with the flat pliers, the end covered with soldering 

 fluid and dipped into the lead. The pliers must be used because 

 the metal rod is too hot to be held in the hand after its end has 

 been a short time in the molten lead. The rod must be held up- 

 right until the lead has become solid. The small quantity of lead 

 pushed out by the rod is first cut off with a knife, and then the 

 whole smoothed, using successively the rasp, the smooth file, and 

 emery paper. 



Solid brass balls with iron or brass stalks attached to them may 

 be obtained at the ironmonger's, and with a little trouble they may 

 be prepared so as to form knobs which look much better than the 

 kind just described. Besides knobs such as that represented in fig. 

 324, A, there is another kind also sold under the name ' stoll-balls,' 



