Pressure on the Length of Disruptive Discharge in Air. 187 



was arranged so that more pipes than are usually provided for 

 could be attached to it. Its base being removed, the bolt 

 which had held it to the base passed through a hole in the 

 table ; and the nut being screwed up the pump was firmly fixed, 



The Discharging Tubes. — These consisted of two cylindrical 

 glass tubes about 4 feet (1*33 metre) long and nearly 3 inches 

 diameter. At one end of each was a tap, the brass pipe from 

 which ended in a ball which formed one of the discharging 

 terminals. Holes in the side of the brass pipe admitted the air 

 from the tap to the tube. At the other end was a stuffing-box, 

 in which a brass rod slid ; at the end of the brass rod was a 

 point which could either be placed in contact with the ball or 

 withdrawn some 3 feet from it. The end of the rod was kept 

 always in the axis of the tube by means of three little glass 

 arms, which were stuck into an ebonite collar fixed on the 

 discharging rod a little behind the point- The two tubes were 

 supported in a horizontal position, parallel to each other and 

 about 18 inches apart, on four ebonite legs about 18 inches high. 

 The tubes were joined to the air-pump by means of the pipes and 

 taps shown in the figure (Plate III.); which were so arranged 

 that the tubes could be at once connected to each other, to the 

 external air, to a gas-holder, or to the pump. Between the 

 tubes and the pump the metal pipe was cut, and a piece of 

 glass tubing about 18 inches long, well varnished with shellac^ 

 was inserted, so that the electricity might not pass to earth 

 through the pump*. 



When the tubes were shut off from the pump, air could 

 always be let into the glass pipe to prevent the discharge pass- 

 ing to earth inside it, as it would do at low pressures. The 

 distance between the point and ball in each tube was measured 

 as follows. They were placed in contact, and an ink mark 

 was made on the discharging rod just outside the collar of 

 the stuffing-box. When the rod was slid out, the distance of 

 this mark from the collar was equal to the distance between 

 the point and ball. The pressure was given by a U-gauge, 

 about 4 feet high, attached to the air-pump at one end, open 

 to the air at the other. 



The pressure P was given by the formula 



P= | height of barometer} — {difference of level of mercury 



in the two arms of the U } . 



Before being admitted into the tubes, the air was dried by 

 bring drawn through sulphuric acid. When it was desired 



* Mr. Apps informs me that it i,- injurious to the coil to connect either 

 secondary terminal to earth when using long sparks. 



