372 



RECENT PROGRESS IN PHYSICS. 



Fiff- 10. 



from personal observation, and his catalogues give no information 

 upon this point. 



§ 12. Improvement in the gold leaf electroscope. — Andriessen has 

 introduced a contrivance into the gold leaf electroscope, by means of 

 which its sensitiveness, and at the sametime its usefulness, is greatly 

 increased. — (Pogg. Ann., LXII, 493.) 



The glass vessel in which the gold leaves hang, is pierced at about 

 the height of their point of suspension, and 

 through this hole a polishedbrass wire, ah i d e, 

 of ^ to f lines diameter is introduced, fastened 

 where properly insulated, and bent as shown 

 in fig. 10. The plane, which the wire forms, 

 must coincide with the plane of motion of 

 the suspended leaves, so that when they diverge 

 one may move toward h i, and the other toward 

 e d. 



The horizontal distance oi h i from d e 

 should be 1|^ inch ; the length of the gold 

 leaves 2 inches ; their breadth as narrowas 

 possible, about 1 line ; the distance of their 

 lower end from the horizontal wire^ d i, ^ an 

 .inch. 



If electricity be communicated to the wire — for instance, the nega- 

 tive electricity of a smooth piece of cork rubbed on a cloth — the leaves 

 diverge, because the wire acts inductively and attracts the -|- E in the 

 leaves, while the repelled — E is driven back to the knob x. The 

 divergence of the leaves increases somewhat when the knob x is 

 touched by a conductor. 



The apparatus is now prepared to indicate the slightest amount of 

 electricity ; if a very small quantity be communicated to the knob x, 

 the leaves either diverge further or collapse, according to the nature 

 of the imparted electricity ; they will collapse if — E be communi- 

 cated to the knob, ^nd diverge if + E is applied. 



The apparatus is sufficiently sensitive to serve for the fundamental 

 experiment of Volta without a condenser. 



If a plate of zinc be substituted for the knob, its upper surface 

 having been freshly rubbed with powdered pumice-stone, and a simi- 

 larly })repared copper plate be placed on the zinc, when the copper 

 plate is removed^ the gold leaves will diverge. 



If, on the contrary, the copper plate be screwed on in the place of 

 the knob x, the suspended leaves will collapse on the removal of the 

 zinc plate. 



Andriessen observed that, by using a bell-glass electroscope of 

 ordinary dimensions with the induction wire, the experiment never 

 succeeded so well as when he used narrow bottles ; heuce^ for his ex- 

 periments, he employed ordinary bottles with ground stoppers 2 to 2i 

 inches wide, and about 4 inches high. He could give no explanation 

 of this fact. 



It was of great importance for the success of the experiment that 

 the air inside the bottle should be perfectly dry ; to accomplish this, 

 Andriessen made a second hole in a suitable part of the bottle, into 

 which he fitted a glass tube (/) filled with chloride of calcium. But 



