ELECTRIC DISCHARGE WITH THE CHLORIDE OE SILVER BATTERY. 159 
leaving the exhaustion to be completed by the Sprengel ; we have thus obtained, 
by the pumps alone, in tubes 32 inches long and 2 inches in diameter, vacua of 
only 0'002 millimetre pressure, equal to 2 - 6 millionths of an atmosphere — a vacuum 
so perfect that the current of 8040 cells would not pass. The apparatus is in connexion 
with a McLeod gauge,* by means of which pressures to 0 '00005 m.m. can be deter- 
mined. Besides this gauge, the Sprengel and Alvergniat pumps have their own 
gauges, which read to a millimetre. M is a rotating mirror consisting of a four- 
sided prism mounted on a horizontal axis and provided with a multiplying wheel ; 
on each face of the prism is fastened a piece of looking-glass. The reflection of the 
tube in the mirror enables one to examine whether an apparently nebulous discharge 
consists really of strata., "also whether and in what direction there is a flow of strata 
which may appear quite steady to the eye. The observations are facilitated by covering 
the tube with a half cylinder of cardboard having a slit in the direction of its axis 
about yq- inch wide. E is a radiometer attached to the Sprengel ; d, d, a drying 
tube containing sticks of potash used when gas is introduced from a reservoir through 
the Alvergniat. 
In fig. 37, to which is attached a scale of feet and inches to enable a judgment to be 
formed of their dimensions, tubes of various forms are represented. It will presently 
be seen that the resistances of these tubes bear no exact relation to the distance 
between the terminals, but that it is affected greatly by the bore of the tube ; the 
small spectrum-analysis tubes, 83, 93, and 95, a portion of which has a capillary bore, 
offer generally great resistance to the battery current, while much longer tubes, 
1 and 2, of larger bore, offer far less. 
In order to test how much of this depends on the length of the constriction, we had 
made two tubes, 154 and 155, fig. 36, of nearly the same length, 16 inches, and internal 
Fig. 30. 
TUBE 154- 
TUBE 155 
<- t jnr t- 
w x y 
A> 
t> 
diameter rfths of an inch, the residual gas in each case being Carbonic acid, COo. 
The distances between the several terminals of tube 154 are respectively between w 
* (PHI. Mag., Aug. 1874.) When the mercury cistern is raised, a portion of gas at the same pressure 
as that in the tube is shut off at 6, and compressed in the small graduated chamber, a, at the top of the 
bulb, to different degrees, in our gauge, from 4 9 ’ 8 3 to , s '- 2 5 , according as the gas is less or more rarefied ; 
the mercury at the same time rises in the pressure column, p, and its height affords the means of 
determining the pressure of the gas in the tube. Tables have been prepared to give the value of the 
reading by inspection. 
