626 THE LEYDEN JAR. 



of the other hand is quickly brought near the knob, it 

 is discharged; a spark appears which is rather small 

 but loud and bright, and a smart shock is felt, es- 

 pecially in the joints. In a small jar, charged by a 

 glass rod or the electrophorus, the length of the spark is 

 from 5 to 8 mm . With the machine, jars of well insulat- 

 ing glass, having a thickness of 3 mm , may be charged so 

 strongly as to give sparks from 3 to 6 cm long ; it some- 

 times happens under these circumstances that a spark 

 passes from one coating to the other across the insula- 

 ting portion of the glass. With equal charges, a jar 

 of "thin glass gives shorter sparks than one of thicker 

 glass, because in the former case the two electricities 

 are nearer to one another ; the mutual attraction is 

 therefore greater and they consequently retain less 

 tension. When the glass is too thin a spontaneous 

 discharge takes place through the glass in consequence 

 of the increased mutual attraction of the two electricities, 

 and the fracture caused by the passage makes the jar 

 useless, for if charged the electricity passes at once 

 through the hole in the glass from the inner coating to 

 the outer. 



For discharging jars without undergoing the electric 

 shock a 'discharger' is used, represented in fig. 326. 

 It consists of a stout bent wire terminating in knobs and 

 attached to an insulating handle. 



A brass wire, from 25 to 40 cm long and 2 mm thick, is softened in 

 tlie flame, and brass balls, of the kind previously described, which 

 may be left hollow, for the sake of lightness, are soldered to the 

 ends. In the middle the wire is bent into the form shown in 

 dotted lines in fig. 326 : this portion is to be fixed into a brass 

 cylinder. A piece of glass, from 12 to 20 cra long, is broken from a 

 larger rod by first making a deep cut all round with the tri- 





