October 17, 1S95] 



NA TURE 



609 



ing Mire is lifted clear out of the can. The can and the 

 <)uadrants in metallic connection with it are disconnected from 

 the case of the electrometer, and the electrified air is very 

 rapidly drawn away from the can by a blowpipe bellows 

 arranged to suck. This releases the opposite kind of electricity 

 from the inside of the can, and allows it to place itself in 

 equilibrivuii on the outside of the can and on the insulated 

 <]iiadrants of the electrometer in mclallic connection with it. 



S 3. We tried clilTerent lengths of time of electrification and 

 different numbers of needles and tinsel, but we found that one 

 needle and four minutes of electrification gave nearly maxinuim 

 effect. The greatest deflection observed was 936 scale divisions. 

 To find, from this reading, the electric density of the air in the 

 can, we took a metallic disc, of 2 cms. radius, attached to a 

 long varnished glass rod, and placed it at a distance of i '45 cm. 

 from another and larger metallic disc. This small air condenser 

 was charged from the electric light conductors in the laborator)- 

 til a dift'erence of potential amounting to too volts. The 

 insulated disc thus charged was removed and laid upon the roof 

 of the large insulated can. This addition to the metal in 

 connection with it does not sensibly influence its electrostatic 

 capacity. The deflection observed was 122 scale divisions. The 



T X 2- I 



cap.tcity of the condenser is approximately . xi-a' ~T- c' 



The quantity of electricity with which it was charged was 



I 100 I . . ,, , 



i-j- ^ "^00 ~ -ic "^''^'•'"■os'^'"^ """• Hence the quantity to 



give 936 scale divisions was 



4-35 



122 



17637- 



vigorously for two and a half 



The bellows was worked 

 minutes, and in that time all the electrified air would be ex- 

 hausted. The capacity of the can was 16,632 cubic centimetres, 

 which gives, for the quantity of electricity per cubic centimetre, 



' ii — I -06 X 10—*. The electrification of the air in this 

 16,632 



case was positive ; it was about as great as the greatest we got, 

 whether positive or negative, in common air 

 w hen we electrified it by discharge from needle 

 points. This is about four times the electric 

 density which we roughly estimated as about 

 the greatest given to the air in the in.side of a 

 large metal vat, electrified by a needle point 

 and then left to itself, and tested by the poten- 

 tial of a water-dropper with its nozzle in the 

 centre of the vat, in experiments made two 

 years ago and described in a communication 

 io the Royal Society in May, 1894.' 



§ 4. In subsequent experiments, electrifying 

 comnion air in a large gas-holder over water 



receiver ot known elliciency and of known ca))acity in connection 

 with the electrometer. We have not yet mea-sured how much 

 electricity was lost in the pass;ige through the india-rubber tube. 

 It was not probably nothing ; and the electric density of the gas 

 before leaving the gas-holder was no doubt greater, though 

 perhaps not much greater, than w hat it had when it reached the 

 electric receiver. 



I § 7. The efficiency of the electric receivers used was approxi- 

 j mately determined by putting two of them in series, with a 

 paraffin tunnel between them, and measuring by means of two 

 quadrant electrometers the quantity of electricity which each took 

 from a measured quantity of air drawn through them. By per- 

 forming this experiment several times, with the order of the two 

 receivers alternately reversed, we had data for calculating the 

 proportion of the electricity taken by each receiver from the air 

 entering it, on the assumption that the proportion taken by each 

 receiver was the same in each case. This assumption was 

 approximately justified by the results. 



§ 8. Thus we found for the efficiencies of two different 

 receivers respectively 077 and 0-31 with air electrified positively 

 or negatively by needle points ; and 0-82 and 0-42 with carbonic 

 acid gas electrified negatively by being drawn from an iron 

 cylinder placed on its side. Each of these receivers consisted of 

 block tin pipe, 4 cms. long and i cm. diameter, with five plugs 

 of cotton wool kept in position by six discs of fine wire gauze. 

 The great dift'erence in their efficiency was no doubt due to the 

 quantities of cotton wool being different, or differently compressed 

 in the two. 



§ 9. We have commenced, and we hope to continue, an in- 

 vestigation of the efticiency of electric receivers of various kinds, 

 such as block tin, brass, and platinum tuljes from 2 to 4 cms. 

 long, and from i mm. to i cm. internal diameter, all of smooth 

 bore and without any cotton wool or wire gauze filters in them ; 

 also a polished metal solid, insulated within a paraffin tunnel. 

 This investigation, made with various quantities of air drawn 

 through per second, has already given us some interesting and 



y an insulated gas flame burning within it 



I ^ 



with a wire in the interior of the flame kept 



electrified by an electric machine to about 



Oooo volts, whether positively or negatively, 



we found as much as i'5 x io~^ for the electric 



density of the air. Electrifying carbonic acid 



in the same gas-holder, '•uhe/her positively or negatively, by 



needle points, we obtained an electric density of 2"2x io~*. 



S 5. We found about the same electric density (2'2 x 10—') of 

 vegalii'c electricity in carbonic aci<l gas drawn from an iron 

 *:ylinder lying horizontally, and allowed to pass by a U-tube 

 into the gas-holder without Inibbling through the water. This 

 electrification was due probably not to carbonic acid gas rushing 

 through the stopcock of the cylinder, but to bubbling from the 

 liquid carbonic acid in its interior, or to the formation of carbonic 

 acid snow in the j.iassages and its subsequent evaporation. 

 When carbonic acid gas was drawn slowly from the liquid car- 

 lionic acid in the iron cylinder placed upright, and allowed to 

 pass, without Inibbling, through the U-tube into the gas-holder 

 ■over water, no electrification was found in the gas unless 

 ■electricity was communicated to it from needle points. 



§ 6. The electrifications of air and carbonic acid described 

 ill §§ 4 and 5 were tested, and their electric densities mea- 

 ■sured by drawing by an air pump a measured quantity of the 

 gas'-' from tlie gas-holder through an india-rubber tube to a 



1 " On llie Klcctritic.ilion of.'Xir,' by Lord Kelvin .ind Magnus Maclc-in. 



- The gas-hoider was 38 cms. high and 81 cms. in circumference' Ten 

 strokes of tlie pump raised the water inside to .1 height of 8"i cms., so that 

 the volume of .lir drawn through the receivers in the experiments wa.s 428 

 .•cubic centimetres per stroke of the pump. This aijrees «'iih ilii^ measured 

 «ITectivc volume of the Iwo cylinders of the pump. 



NO. 1355, VOL. 52] 



surprising results, which we hope to describe after we have 

 learned more by farther experimenting. 



§ 10. In addition to our experiments on electric filters we 

 have made many other experiments to find other means for the 

 diselectrification of air. It might be supposed that drawing air 

 in bubbles through water should be very effective for this "pur- 

 pose, but we find that this is far from being the case. We had 

 previously found that non-electrified air dr.awn in bubbles through 

 pure water becomes negatively electrified, and through salt water 

 positively. We now find that positively electrified air drawn 

 through pure water, and negatively electrified air through salt 

 water, has its electrification diminished but not annulled, if the 

 IJrimitive electrification is sufficiently strong. Negatively 

 eleetrified air drawn in bubbles through pure water, and 

 positively electrified air drawn thniugh salt water, has its 

 electrification augmented. 



§ u. To test the eflects of heat we drew .air through com- 

 bustion tubes of German glass about iSocms. long, ami i\ or \k 

 cms. bore, the heat being applied externally to about 120 cms. of 

 the length. We found that, when the temperature was raised to 

 nearly a dull re<l he.at, air. whether positively or negativclv 

 electrified, lost little or nothing of its electrification by being 

 drawn through the tube. When the temperature was raised to a 

 dull red heat, and to a bright red, high enough to soften the 



