50-1. HEPORT — 1867. 



bringing the potential of the chief electrode and upper attracting-plate &c. 

 all to the potential of the air at the point where the match burns*. The 

 instrument is either held in the observer's hand, or it is placed upon a fixed 

 support, and care taken that its outer brass case is in connexion with the 

 earth. When the diiference of potentials between two conductors is to be 

 tested, one of these is connected with the brass case of the instrument, and 

 the other with the chief electrode, the umbrella being kept up. If both 

 of these condiictors must be kept insulated from the earth, the brass case of 

 the electrometer must be put on an insulating stand, and the micrometer- 

 screw turned by an insiilating handle. 



§ 28. A lead cup {e e, fig. 8), supported bj metal pillars from the roof and 

 carrying pieces of pumice-stone, held in their place by India-rubber bands, 

 completes the instrument. The inner surface of the glass must be clean, and 

 particles of dust, minute shreds or fibres, &c. removed as carefully as possible, 

 especially from the lower surface of the upper attracting-plate, and the ujiper 

 surface of the guard-plate and aluminium square facing it from below. The 

 pumice is prepared by moistening it with a few drops of strong pure sulphiirio 

 acid. Ordinary sulphuric acid of commerce should be boiled with sulphate of 

 ammonia to free it from volatile acid vapours, and to strengthen it sufficiently 

 by removing water if the acid be not of the strongest. There should not be 

 so much acid applied to the pumice as to make it have the appearance of 

 being moist, but there must be enough to maintain a sufficiently dry atmo- 

 sphere within tlie instrument for very perfect insulation of the Leyden jar, 

 which I find does not in general lose more of its charge than 5 per cent, per 

 week, when the pumice is properly acidulated. Thus there is no tendency 

 of the liquid to drop out of the pumice ; and the pumice being properly 

 secured by the india-rubber bands, the instrument may be thrown about 

 with any force, short of that which might break the glass jar or either of 

 the glass stems, without doing any damage ; but to ensure this hardiness 

 the sheet aluminium of which the balance is made must be very thin. After 

 several weeks' use the jmmice may commence to look moist, and even slight 

 traces of moisture may be seen on the outside of the lead cup, in conse- 

 quence of watery vapour attracted by the sulphuric acid from the atmosphere ; 

 but the piimicc should then be taken and dried. At all events this must be 

 done in good time, before enough of liquid has collected to give anj' tendency 

 to drop. In all climates in which I have hitherto tested the instrument, 

 I have found the piimice cftcctive for insulation and safe in keeping all the 

 liquid to itself for two months. But it having been reported to me by Mr. 

 Becker that many instruments have been returned to him in a ruinous con- 

 dition from drops of sulphuric acid having become scattered through their 

 metal work, I now cause to be engraved conspicuously on the outer case of 

 the instrument " pumice dangekous, if not deied once a MONin ;" also a 

 frame carrying a card, on which the dates of drying are inscribed, to bo 

 placed in a convenient position on the roof of the instrument. 



§ 29. To prep;irc the instrument for nse, the inner coating of the Leyden. 

 jar must be charged through a charging rod, insulated in a vulcanite or 

 glass tube, and let down for the occasion through a hole in the roof of the 

 instrument, by aid of a small electrophorus, which generally accompanies 

 the instrument, or hy an electrical macliiue. I generally prefer to give a 

 negative charge to the inner coating, as I have not found any phy- 

 sical reason, such as that mentioned in § 9 above, to prefer a positive charge 



* See Nichol's Cyclopedia, article " Electricity, Atmospheric," 2nd edition, 1860 ; or 

 "Boyal Institution Lecture on Atmospheric Electricity," May 1860. -- 



