4 TRANS. OF THE ACAD. OF SCIENCE. 



the collected electricity were rather defective, especially 

 when it hecame necessary to observe electricity of very fee- 

 ble tension. As measuring- instruments for this purpose, there 

 were used, at first, only two single threads, or little balls of the 

 pith of elder, or the attractive power of the electrified wire 

 for sawdust, bran and cotton, or the Ley den phial, or a straw 

 or goldleaf electrometer, simple or graduated, or a dry 

 (zambonic) pile; or, later, since the discovery of electro- 

 magnetism, a needle galvanometer (rheometer). But none 

 of these instruments proved to be sensitive enough for meas- 

 uring very feeble degrees of atmospheric electricity, and none 

 therefore fully answered the purpose of making regular daily 

 and hourly observations in all kinds of weather. The most 

 delicate instrument of that class was probably Coulomb's 

 electroscope, a torsion balance, in which a needle of shellac, 

 attached to a cocoon thread, is moved by the smallest electri- 

 cal force and turned round over a graduated circle. This 

 instrument was originally intended merely for examining the 

 electrical properties of various bodies after friction, and proved 

 to be the most sensitive and practical one for that purpose; 

 but the idea of such a torsion balance for electrical measure- 

 ment has been taken up again in our day by Prof. F. Dell- 

 mann, of Germany, and, by his many and ingenious improve- 

 ments, it has been rendered the most delicate, exact and 

 practical instalment for regular observations of atmospheric 

 electricity that has come to my notice. Prof. Dellman is a 

 scientific man in Kreuznach, on the Rhine, and, for several 

 years past, he has made there regular daily observations of at- 

 mosjmeric electricity for the Prussian State, which publishes 

 them in the annual reports of the Royal Meteorological In- 

 stitute in Berlin. 



Dellmann's electrometer is a torsion balance. A very fine 

 glass thread, about eighteen inches long, running vertically 

 through a glass tube, has, on its lower end, fastened to it by 

 shellac, a very thin and light beam or needle of brass in a 

 horizontal position. This beam, when moved by any force, 

 will be driven around in a circle over a metallic disc with a 

 graduated scale, divided into four times ninety degrees. Di- 

 rectly below the movable beam, but unconnected with it, is 

 another similar thin piece of brass, which is fixed, perfectly 

 isolated from the metallic disc, and ends below in a metallic 

 wire, also isolated, to which the electrical charge is applied 

 from without. The upper beam with the glass thread can, by 

 a micrometer screw, be lowered, or elevated, so as either to 

 touch the lower beam, or hang suspended above it. By 

 another ingenious mechanism, the upper beam, when at rest, 

 is always kept exactly over the lower one ; and the three 

 iron feet, upon which the whole instrument rests, can be 

 screwed up and down so as to give it a proper level. Now, 



