56 Laboratory Arts 



" round brass i^" long is required, and half this should be filed 

 taper to slightly under a \" diameter (Fig. 51). This will be more 

 easily done on a lathe, of course ; failing this, the brass rod should 

 be held in the left hand and rotated in an inclined direction on a 



FIG. 50. 



FIG. 51. 



wooden support, one end only of the rod touching the support, the 

 file being used in the ordinary way, on the rotating rod. Care must 

 be taken to keep the rod rotating one complete revolution in order 

 that the taper may be symmetrical, and if this be done little difficulty 

 will be found in tapering the plug correctly. It is filed in this way 

 until it enters each hole and makes good contact without rocking. 

 When this is the case, the plug is fitted with a brass wire handle, 

 smeared with emery powder and lubricating oil, and ground into 

 its place by a spiral twisting motion, the emery being renewed fre- 

 quently. The plug will gradually wear away from the mounted 

 brass pieces a place for itself. The plug should be frequently 

 changed from one hole to the other, in order that both holes and 

 plug may arrive at the same shape together. As soon as the plug 

 fits well, and without shaking, the grinding may be discontinued 

 and the instrument cleaned up. 



If the terminals sketched are used, they will not be in the way 

 of the plug, but bought terminals may, consequently it is well to 

 make sure of the sizes before cutting the plug. 



EXERCISE 4. Tin plate condenser jacket. 



Tin plate is not a good material to use for this purpose, but one 

 can frequently make one up more quickly than in glass, which 

 takes a long time to anneal, and even then is usually more fragile 

 than a tin one. Copper, of course, would be a superior metal, and 

 may be treated in exactly the manner indicated below, a permanent 

 and useful piece of apparatus being thus obtained, whereas a tin- 

 plate jacket only lasts a short time before rusting. Tin plate, 



