MAGNETIC EFFECTS OF THE DISCHARGE. 643 



cavity in the wooden block and 10 or 15 mm long, is placed 

 upon the mouth of the cavity. When the discharge 

 takes place the cork is either lifted for a moment by 

 the disturbed air or thrown entirely off. 



Another effect may here be briefly mentioned; more 

 will be said about this subject farther on. If the dis- 

 charge is directed through a wire which is coiled round 

 a small bar or needle of hard steel, the needle becomes 

 magnetic, that is, it acquires the property of attracting and 

 supporting small particles of iron, such as iron filings. 



A wire of copper, or brass which has been softened in 

 the flame, is spirally wound round a glass tube, as in 

 fig. 334, the coils (twenty or more) being about l mm> 5 



iG. 334: (real ske). 



distant from each other. A short piece at both ends of 

 the wire is left to hang straight down, and^ both extre- 

 mities are bent into rings, to which chains may be hung 

 for connecting the wire with the external coating of a 

 jar at one side and with thq knob of the discharger at the 

 other. The free part of the glass tube (on the left of 

 the figure) is clamped in. the retort-stand, and a knitting 

 needle or stout darning needle is placed inside the tube 

 within the portion round which the wire is coiled. 

 When the discharge has taken place, and the needle is 

 taken out, it will be sufficiently magnetic to attract a 

 few iron filings. 



Needles purchased in the shops are sometimes already magnetic ; 

 but if the needle used be tested before the experiment, by holding 



T T 2 



