624 THE LEYDEX JAR. 



as shown in fig. 121. To give a better appearance to it, tne 

 or disc may be covered with a coating of a solution of 12 grammes 

 of sealing-wax in 6 CC of spirit of wine, made in a small corked glass 

 bottle. Abont a day is required for the sealing-wax to dissolve, but 

 frequent stirring is required during this time and also previous to 

 using it, as it is apt to be lumpy. The coating dries very slowly ; a 

 few days elapse before it is quite hard. It is usually necessary to 

 repeat the operation several times, but no new coating should be 

 given unless the previous one is perfectly dry. 



For very large jars two elastic brass strips, placed crosswise, will 

 be required to secure the steady position of the rod. 



The most convenient mode of charging the Leyden jar is to place 

 the left hand round the lower portion of the outer coating, and hold 

 the jar so that the knob is quite close to, or touches, the conductor of 

 the machine, which is worked with the right hand. Large jars 

 which cannot be safely held with one hand should be held with both 

 hands, the machine being worked by an assistant ; or they may be 

 placed near to the machine upon the table, and conductor and knob 

 joined by the discharging rod described farther on. 



Jars of medium or small size may be charged from the electro- 

 phorus by sending 50 or 100 sparks from the cover into the knob. 

 The electrophorus should for this purpose be raised on some support, 

 so as to be only 5 or 6 cm below the knob of the jar ; it is then not 

 needful to lift the cover very high. The following mode is still 

 more convenient : The jar is held in the left hand nearly upside 

 down (this of course cannot be done with the test jar filled with 

 iron filings), a little slanting, the knob being about 4 or 5 cm above 

 the edge of the cover of the electrophorus. The strings of the 

 cover being held between the thumb and forefinger of the right 

 hand, the little finger of the same hand is allowed to touch the 

 cover while it still rests on the cake ; then the thumb and fore- 

 finger are moved upwards until the cover sends a spark into the 

 knob of the jar, the cover is then again lowered until it touches the 

 cake, and thus by alternately touching with the little finger, and 

 raising and lowering the two other fingers, the jar may be very 

 conveniently charged. Care must be taken not to touch the cover 

 with the hand while it is near to, or in contact with, the knob of 

 the jar ; in that case the jar would be discharged, and the unex- 

 pected shock might cause the experimenter to drop the jar and 

 break it. 



A very small jar may be charged by the glass rod. The jar- 

 being held between the outstretched thumb and forefinger of the 

 left hand, the cloth with the amalgam is kept round the glass rod 



