ELECTRICITY. 



517 



Practical 

 Electricity. 



On thunder 



Feed's 



thunder 



rod. 



PLATE 

 CCXLVIII. 

 Fig- 10. 



Tled'i 

 hand ex- 

 ploring rod. 



SECT. II. On Thunder Rods. 



IN our Section on the history of the atmosphere, -we 

 have already given an account of the simple thunder 

 rods which were employed by Le Monnier, Mazeas, 

 Beccaria, Dalibard, and'Delors. We shall therefore, 

 describe at present the more complicated instruments 

 constructed and used by Mr Read, Mr Cavallo, and Mr 

 Ronalds. 



The instrument employed by Mr Read, in the expe- 

 riments of which we have already given a full account, 

 is represented in Plate CCXLVIII. Fig. 10, where AA 

 is a wooden rod 20 feet long, and 2 inches in diameter 

 below, and 1 above. A solid glass pillar B covered 

 with wax, 22 inches lon:r, is cemented into the lower 

 end of it, and along with the whole rod is supported 

 upon a wooden pedestal C, at the end of an iron brac- 

 ket driven into the wall. At the distance of lii inches 

 from D, a strong arm of wood E is fixed into the wall, 

 holding a glass tube F, covered with sealing-wax. 

 The rod A A passes through the tube, and thus stands at 

 the distance of 12 inches from the wall. The tube F is 

 lined witli cork, so that when the rod is bent by the 

 wind, it is thus prevented from touching or breaking 

 the tube. At the upper end of the rod are several 

 sharp pointed wires G, two of which are of copper, and 

 about one-eighth of an inch thick, and one of them is 

 twisted round the rod to the right, and the other to the 

 left, so as to reach the brass collar at the top of the 

 lower funnel H, to which they are soldered. The two 

 funnels H, H, are intended to defend the glasses B, F 

 from the weather. A hole is Irored through the wall 

 at I, to receive. a glass tube coated witli sealing-wax, 

 through which a strong brass wire is conveyed from 

 the roil into the room. At the end of the glass tube it 

 paMes through a brass ball 2 inches in diameter; and 

 proceeding a little farther, it keeps suspended at its ex- 

 tremity a pith ball electrometerK.at the distance of about 

 1-J inches from the wall. The outer end of the glass 

 tube is kept dry, by a wooden box on the outside of 

 the wall. A bell N, supported on a strong wire, is pla- 

 ced at the distance of 2 inches from the ball L, and the 

 wire of the bell communicates by a metallic continua- 

 tion II with the moist ground. This bell is rung by a 

 small brass ball three-tenths of an inch in diameter, 

 suspended from a nail at O. A small table P is erect- 

 ed, for holding jars and other pieces of apparatus for 

 making experiments. Mr Read afterwards brought all 

 the insulated parts of this rod under the roof of his 

 house, to preserve them from the moisture of the atmo- 

 sphere. 



When the insulation of his principal rod was so far in- 

 jured by the moisture of the air that it exhibited no elec- 

 trical indications, he made use of what he calls a hand ex- 

 ploring rod, which was about the length and thickness of 

 a common fishing-rod, having plenty of small wires 

 twisted round it from one end to another. In order to use 

 this rod, he warmed the glas* legs of an insulating stool, 

 and having placed himself upon it, he raised with one 

 hand the rod into a vertical position, where he kept it for 

 a minute or two. With the finger of the other hand he 

 touched a sensible electrometer, which pointed out the 

 electricity of the atmosphere. When the electricity was 

 too weak to produce any divergency in the pith balls, 

 which did not often happen, he added to the rod a light- 

 ed torch, and placed it as far from his hand as the 

 strength of the rod would permit ; and by repeating the 

 experiment, he always found the electricity sensible, 



Cavallo's 

 atmospheri- 

 cal collec- 

 tor. 



PLATE 

 CCXLVIII. 



Fig. 11. 



Mr Singer's 

 atmospheri- 

 cal appara- 

 tus. 



being attracted more powerfully by the flame of the Practical 

 torch than by the extremity of the rod. Electricity.^ 



The atmospherical collector, with which Mr Cavallo 

 made numerous experiments, is represented in Plate 

 CCXLVIII. Fig. 11, -where A B is a common jointed 

 fishing-rod, which wants the last or smallest joint. A 

 slender glass tube C is fixed at the extremity of this 

 rod, and is covered with sealing-wax, having a cork D 

 at its end, from which a pith ball electrometer is sus- 

 pended. A piece of cord HGI is fastened to the other 

 extremity of the rod H, and is supported at G by a 

 small piece of twine FG. A pin is fastened at the ex- 

 tremity I of the cord, and when it is pushed into the 

 cork D, the pith ball electrometer is in an uninsulated 

 state. In order to observe the electricity of the atmo- 

 sphere, he fixed the pin in the cork D, and taking 

 the rod by the lover extremity H, he held it out of one 

 of the highest windows in the house, and having raised 

 it into a position inclined about 50 or G0 to the hori- 

 zon, he kept it there for a few seconds ; he disengaged 

 the pin from the cork D, by pulling the twine at H ; 

 and the string dropping into the dotted position KL, 

 left the electrometer insulated and electrified in a state 

 opposite to that of the atmosphere. He then drew the 

 instrument into the room, and examined the character 

 of the electricity which it possessed. 



In order to insulate a vertical atmospherical appara- 

 tus, Mr Singer has applied his new system of insulation, 

 of which we shall give an account in our description of 

 his electrometer. He proposes to employ a stick of 

 glass 10 inches long, and one inch in diameter, coated, 

 with sealing-wax. The brass cap at each extremity, 

 must have a screw to receive the lid of a cylindrical tin 

 funnel. " There are to be two such funnels, one screw- 

 ed at each end of the insulating pillar. They may be 

 about 8 inches long, and one smaller than the other, in 

 such proportion, that the circumference of the stick of 

 glass, and the two funnels, may form a series of concen- 

 tric circles, distant from each other about a quarter of 

 an inch. The apparatus is represented by Fig. 12, the 

 funnels being delineated by dotted lines. It is evident 

 that in this apparatus, the vapour must first traverse the 

 sp.u-e between the outer and inner funnels, and then 

 the interval between t'ie inner tunnel and the stick of 

 glass, before the insulation can be destroyed; and this 

 space may be lengthened to any extent, by increasing 

 the number of concentric tunnels. This arrangement 

 is very simple and durable; and though the limit of its 

 insulation is the distance of the funnels, that is, a quar- 

 ter of an inch, this will be found sufficient for the most 

 essential observations on the atmosphere ; and the high- 

 er intensities may be obtained, if desired, by prolonging 

 the insulator to some inches below the cap of the lower 

 funnel, as shown in the Figure, or by making this lower 

 and internal funnel of a glass tube covered with seal- 

 ing-wax. 



If an apparatus of this description be used to insu- 

 late the horizontal wire, the open end of the larger fun- 

 nel should have a circular tin plate of nearly twice its 

 diameter, placed opposite to it, at a short distance, to 

 prevent the intrusion of driving rain or snow. (Fig. 

 13.) Or, what might perhaps prove more effectual, it 

 may be placed within a sort of pigeon house, having a 

 hole in its side for the wire to pass through " 



Nearly upon the preceding plan of insulation, F. Ro- 

 nalds, lisq. of Hammersmith, has erected an atmo- 

 spherical apparatus, in a field near Highbury Terrace, 

 Islington ; but we have not yet seen an account of the 

 observations which he has made witli it. See Read, 



Fig 1?: 



Fig. 13. 



Ronalds' 

 thunder 

 rod. 



