RECENT PROGRESS IN PHYSICS. 



377 



turned upon its axis once or twice, so that the hall a, hy means of the 

 elasticity of tlie thread, is hrought against the ball /9. 



Ih'llman subsequently changed the construction of his instrument, 

 and tluis made it much more sensitive. Fig. 13 represents Dellman's 

 electroscope in its new form, (Pog. Ann. LVIII, Fig. 13. 



49.) The moveable beam cj consists of a light ^ — 

 metallic wire, which is bent in tlie middle so that — T-iL — 

 one-lialf of the balance beam can be placed on the 

 right, and tlie other on the left side of the me- 

 tallic strip /. This strip /extends through the 

 middle of the apparatus, and is fastened on one 

 side to the conducting wire h, on the other to the 

 wire e. 



If electricity be communicated to the conduct- 

 ing wire (on which a plate, such as that of a 

 condenser, can be screwed) it will, in part, pass to the balance beam, 

 (pressed by the torsion of the fibre against the metal strip /,) which 

 is, consequently, deflected. 



One vertical arm of the wire, h, whose horizontal part makes an 

 angle of about 90° with the direction of the strip /, is on the right, 

 and the other vertical arm on the left side of the beam. Dellman 

 terms this wire, to which electricity can be communicated from above, 

 and whose function is the same as the wire in the electrometer of 

 Andriessen, the "cross wire." When electricity is communicated to 

 the cross wire, the beam g at once turns and stands in a new position 

 of equilibrium. It moves to one or the other side according to the 

 kind of electricity communicated to the wire h. It is evident that the 

 apparatus in this form must be exceedingly sensitive; but it is very 

 troublesome, as the beam suspended by the silk fibre commences vi- 

 brating with the least disturbance. 



The paper alluded to in Poggendorf's Annalen, in wliich Dellman 

 speaks of the new form of his electroscope, is somewhat obscurely 

 written. A particular description of the apparatus or of its applica- 

 tion is not to be found. The proper dimensions for the strip and the 

 best position for the cross-wire are spoken of without any allusion 

 having been previously made to the cross-wire and strip. Evidently, 

 Bellman takes for granted much that is not familiar to most of his 

 readers. 



The arrangement of the strip / is evidently somewhat awkward. 

 liomerhausen has very advantageously altered it. The balance beam, 

 which is of common flat gold wire, is straight, while the metallic strip 

 /"lias a bend in the middle, as represented in figure 14. 



The metal strip is fastened at its middle 

 to the conducting wire. In this instrument 

 the torsion of the silk fibre is opposed to the 

 repulsion of the beam, while in Oersted's elec- 

 trometer the magnetism of a small iron wire 

 tends to keep the beam in a given position. 



Fis. 14. 



