through Exhausted Tabes without Electrodes. 323 



fig. 1 . Two jars were used, the outside coatings of which, A and 

 B, were connected by the wire containing the primary coil C, 

 the inside coating of the first jar was connected to one pole 

 of the Wimshurst, that of the second to the other. With this 

 method of arranging the jars no air-space is required, as the 

 sparks pass between the terminals of the machine, and the 

 polishing of these terminals is not nearly so important as that 

 of the knobs of the air-break in the arrangement (x). 



Before proceeding to describe the appearance presented by 

 the discharge, I will mention one or two points which may 

 prove useful to any one who wishes to repeat the experiments. 

 According to my experience the discharge is more easily 

 obtained in bulbs than in square tubes, and with a Wimshurst 

 machine than with an induction-coil. If an induction-coil is 

 used a break which will transmit a large current ought to 

 be substituted for the ordinary vibrating one supplied with 

 such instruments. It is essential to success that the gas 

 in the bulbs or tubes should be quite dry and at a suitable 

 pressure ; there is a pressure at which the brilliancy of 

 the discharge is a maximum, and as in endeavouring to 

 get at this pressure the exhaustion may be carried too far, 

 it is convenient to use a form of mercury pump which will 

 allow of the easy admission of a little gas ; the pattern 

 which I have used and found to answer very well is called 

 the Lane-Fox pattern. When any gas is introduced it 

 should be sent through sulphuric acid to get rid of any 

 moisture that may be in it. Owing, I think, to the pressure 

 in ordinary incandescent lamps being very different from that 

 at which the discharge has its maximum brilliancy, I have 

 met with very poor success in attempts to produce these 

 discharges in already exhausted tubes such as incandescent 

 lamps, though I have tried a considerable number by different 

 makers ; on the other hand, the radiometers which I have 

 tried allow the discharge to pass pretty readily, though it 

 is interfered with by the vanes, and is not comparable in 

 brilliancy with that obtained in home-made tubes and bulbs. 

 I have obtained sparks easily with apparatus of the following 

 dimensions : two gallon jars, the outside coatings connected 

 by a wire about 2 yards long, the coil consisting of three or 

 four turns, each about 3 inches in diameter. I have some bulbs 

 which with this apparatus will give a bright discharge when 

 the distance between the terminals of the Wimshurst is only 

 J inch ; these are, however, exceptionally good ; it more 

 frequently takes a spark an inch or an inch and a half long- 

 to produce the discharge. 



I find that Hittorf in Wiedemann's Annalen, xxi. p. 138, 



Z2 



