378 



RECENT PROGRESS IN PHYSICS. 



The most important parts of Oersted's instrument are sliown (one- 

 half the natural size) in figure 15. In the 



Fi? 



cover of a glass vessel is a glass tube d d, in 

 which a metal tube, insulated with shellac, is 

 fastened and divided below into two bent arms, 

 c c. In the middle of this tube hangs the silk 

 fibre which carries the beam a a, made of a 

 fine brass wire ; ft & is a stirrup of very fine 

 iron wire, very slightly magnetized, by means 

 of which the beam is pressed against the brass 

 arms c c, so that one end of a a touches the left 

 side of c c and the other end the right side. 

 When electricity is communicated from above 

 it is conducted from the arms c c to the balance 

 beam a a, causing it to turn. If the magnetic 

 directive power is very feeble the electrometer 

 will possess greatsensibility. To discover weak electrical effects, enough 

 electricity is previously communicated to the instrument to cause the 

 beam to diverge a few degrees. A substance, possessing the same kind 

 of electricity, then produces a considerable increase of the deviation 

 when brought near. The electricity, which insulated zinc and copper 

 plates show, upon contact and separation, is thus rendered very per- 

 ceptible, without the aid of a condenser. DeUman's instrument also 

 shows this fundamental experiment of VoUa without a condenser. 



Oersted adapted to his instrument a contrivance for measuring 

 the angle of deflection, and a microscope to observe the position of the 

 beam more accurately, &c. I have omitted these as not necessary 

 in explaining the principle of the instrument. Oersted himself used 

 the instrument only as an electroscope. 



Koldrausch has converted DeUman's electroscope into an electro- 

 meter. — (Pogg. Ann, LXXII, 353.) He introduced under the beam a 

 divided circle for reading the angle of deflection, and a second, at the 

 top of the instrument, for determining the torsion. Instead of cocoon 

 fibre he used a fine glass thread, because its force of torsion is more 

 reliable. 



DeUman's instrument has this great advantage, that it may be 

 constructed with but a few and common materials, so that any one 

 having but little dexterity can make such an instrument for himself. 

 This advantage of DeUman's apparatus Kohlrausch has surrendered 

 altogether, for his instrument can be made only by a skilful me- 

 chanic. However, if the a})paratus in this form has advantages which 

 it had not in its simpler form, no objection can be made. According 

 to Kohlrausch' s memoir, his electrometer serves for accurate measure- 

 ment in cases for which the torsion balance of Cquloiyib is not suffi- 

 ciently sensative. I cannot give a decide! oi)inion as to the value of 

 Kohlrauscli' s electrometer in this respect, for I have not ex]-)erimented 

 with it. I do not know wliether the same amount of trouble is found 

 in its use that must be required in its construction. The instrument 

 seems to me to be rather comi)licated ; but whether this view is well 

 founded I must leave to the judgment of those who have made prac- 

 tical use of it. Tlie results which Kohlrausch presented in his memoir 

 are much in favoi of his instrument. 



