WISLIZENUS — ATMOSPHERIC ELECTRICITY. 5 



if an electrical charge is applied to the outside wire and to 

 the lower beam, and the upper beam, by screwing it down, 

 is for a moment brought in slight contact with the lower one, 

 the upper beam will be charged with the same electricity ; 

 and, since equal electricities repel each other, the movable 

 upper beam will be driven off a certain number of degrees, 

 according to the intensity of the charge. If both beams 

 were movable, both would be driven in opposite directions, 

 like the two leaves of a goldleaf electrometer ; but, as one 

 beam is fixed, the other movable one can alone give way, 

 and, by its motion, indicates the force of the electrical charge. 

 The box, containing the graduated disc and the beams, is 

 covered with a glass to protect them from dust and dampness, 

 and, with the aid of a loiqye, the grades can be easily read. To 

 determine the positive or the negative character of the elec- 

 tricity, immediately after reading the degrees, the outside 

 wire is again charged with electricity of a known quality, 

 (for instance, with the negative electricity from a resinous 

 cork, rubbed over woollen cloth.) If the first electricity is 

 of the same character as the second, the movable beam will 

 be still more repelled, because like electricities repel each 

 other; if of opposite character, the movable beam will, at 

 once, return to the fixed one, because unlike electricities at- 

 tract each other. 



Since glass threads, however fine they may be, are never 

 quite equal, and a slight difference in the glass threads may 

 cause slight variations in the instrument, it is needed, for 

 comparative observations on different instruments, to deter- 

 mine the sensitiveness of each instrument by the tension of a 

 zinc-copper column of a certain size, the tension of one ele- 

 ment to be used as unity for the table of calculation. 



For collecting atmospheric electricity, Prof. Dellmann uses 

 also a peculiar apparatus. He believes, with many others, 

 that a fixed apparatus does not answer the purpose, because it 

 can never be sufficiently isolated, is charged but slowly and 

 gradually, and does not indicate the actual electricity of the 

 surrounding atmosphere. He therefore makes use of a 

 movable apparatus, that is, a hollow ball of copper or brass, 

 six inches in diameter, with a metallic stem ; the latter rest- 

 ing in a metallic tube, from which it is perfectly isolated by 

 shellac. This collecting apparatus is fixed to a pole about 

 thirty feet long, which is, by a windlass, drawn up along the 

 wall of the house to the height of the roof, where the air 

 circulates freely ; the observer then touches, by means of a 

 thin wire, the stem of the ball with a half-moon-shaped piece 

 of brass ; the ball is thus charged with electricity, the pole 

 let down again quickly, and the collecting apparatus is, at 

 once, brought into contact with the measuring instrument. 



The electricity, thus collected in the ball, is not directly 



