1882.] Manometric Observations in the Electric Arc. 265 



spectrum. "While the arc is in this unstable condition manometric 

 observations are impossible, as the small area of the carbon tube is 

 rarely completely covered by the arc, so that the manometers often 

 record neither positive nor negative pressures during the discharge. 

 The effect of the hot poles on the registration of the manometers is to 

 produce a small negative pressure when the arc has stopped, due 

 to the passage of currents of hot air. Many experiments were made 

 to ascertain if a local heating of the carbon tube caused any per- 

 manent pressure. This was carefully tested by taking the arc at right 

 angles to a carbon tube placed in a block of magnesia so as to 

 raise the middle of the tube to the highest possible temperature. 

 Under these conditions the manometer connected with the hollow 

 carbon remained perfectly steady, whether the tube was made the 

 positive or negative pole of the battery. This experiment also 

 showed that repulsion of the inclosed gas in the tubes through an 

 electric charge, had no effect on the manometer. During the main- 

 tenance of the steady arc, the manometer connected with the positive 

 pole exhibits a fixed increase of pressure, corresponding to a motion of 

 the fluid in the horizontal tube of the manometers employed, of 

 from 50 to 150 millims., which is equivalent to from 1 to 2 millims. 

 of vertical water pressure, in different experiments and under varied 

 conditions. The manometer connected with the negative pole shows 

 no increase of pressure, but rather on the average a diminution. 



When the arc begins to emit a hissing sound, the pressure on the 

 positive pole instantly diminishes, and when blasts are ejected from 

 the positive in the direction of the negative, the negative manometer 

 which had stood at zero before showed a marked increase of pressure. 

 If a commutator is placed in the circuit, so as to quickly reverse 

 the direction of the original current, the arc is not broken, but the 

 manometers immediately record a reverse action. In order to equalise 

 the temperature of the poles to some extent, the arc was taken in the 

 middle of a block of magnesia, but the same results were observed ; 

 the pressure is generally smaller in the hot crucible than it is in air. 

 When the crucible gets highly heated and filled with metallic vapours, 

 the arc will pass a distance of more than an inch, and in this con- 

 dition the shorter the arc the greater the pressure. It was found 

 advisable in these experiments to use a negative pole which had a 

 sharp conical termination, otherwise the form of the arc in the block 

 of magnesia was very irregular, owing to the high conductivity of the 

 hot walls of the crucible. When the poles are brought into contact in 

 the magnesia crucible, the pressure at the positive instantly falls. 



Whether air, carbonic oxide, or nitrogen filled the manometer and 

 carbon tubes the results were invariably the same. The chief experi- 

 ments have all been made with the Siemens machine, but a 70-cell 

 Grove's battery produced the same results. The use of tbe horizontal 



