148 ANNUAL OF SCIENTIFIC DISCOVERY. 



having been, as it were, protected by electrization from the action of light. 

 The figures were permanently fixed by a strong solution of hyposulphate of 

 soda. 3 



o 

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OX A NEW METHOD OF OBSERVING ATMOSPHERIC ELECTRICITY. 



At the British Association for 1856, Professor W. Thomson gave the fol- 

 lowing description of a method of observing atmospherical electricity, em- 

 ployed by Mr. Dellman, of Crenznach, Prussia, Mr. D. being employed 

 by the government to make meteorological observations. It consists in using 

 a copper ball about six inches diameter, to carry away an electrical effect 

 from a position about two yards above the roof of his house, depending 

 simply on the atmospheric "potential " at the point to which the centre of 

 the ball is sent : and it is exactly the method of the " carrier ball " by which 

 Faraday investigated the atmospheric potential in the neighborhood of a 

 rubbed stick of shell-lac, and other electrified bodies (" Experimental Re- 

 searches," series ix, 1839.) The whole process only differs from Faraday's 

 in not employing the carrier ball directly, as the repeller in a coulomb-elec- 

 trometer, but putting it into communication with the conductors of a sepa- 

 rate electrometer of peculiar construction. The collecting part of the ap- 

 paratus is so simple and easily managed that an amateur could, for a few 

 shillings, set one up on his own house, if at all suitable as regards roof and 

 windows ; and, if provided with a suitable electrometer, could make obser- 

 vations in atmospheric electricity with as much ease as thermometric or ba- 

 rometric observations. The electrometer used by Mr. Dellman is of his own 

 construction (described in Poggendorff 's "Annalen/ 1 1853, vol. 89, also 

 vol. 85), and appears to be very satisfactory in its operation. It is, I be- 

 lieve, essentially more accurate and sensitive than Peltier's, and it has a 

 great advantage in affording a very easy and exact method for reducing its 

 indications to absolute measure. I wish also to suggest two other modes of 

 observing atmospheric -electricity which have occurred to me, as .possessing 

 each of them some advantages over any of the systems hitherto followed. 

 In one of these I propose to have an uninsulated cylindrical iron funnel, 

 about seven inches diameter, fixed to a height of two or three yards above 

 the highest part of the building, and a light, movable continuation (like the 

 telescope funnel of a steamer) of a yard and a half or two yards more, 

 which can be let down or pushed up at pleasure. Insulated by supports at 

 the top of the fixed part of the funnel, I would have a metal stem carrying 

 .1 ball like Dellman's, standing to such a height that it can be covered by a 

 hinged lid on the top of the movable joint of the funnel, when the latter is 

 pushed up ; and a fine wire fixed to the lower end of the insulated stem, and 

 hanging down, in the axis of the funnel to the electrometer. When the ap- 

 paratus is not in use, the movable joint would be kept at the highest, and 

 its lid down, touching the ball so as to keep it uninsulated. To make an 

 observation, the lid would be turned up rapidly, and the movable joint car- 

 rying it let down, an operation which could be effected in a few seconds by a 

 suitable mechanism. The electrometer would immediately indicate an in- 

 ductive electrifi cation simply proportional to the atmospheric potential at 



