LIPPMANN'S GALVANOMETER. 



243 



on the same principle, which is equally remarkable for the rapidity of 

 its damping (Fig. 165). The frame, which is rectangular, is sus- 

 pended by two metal wires which convey the current, and the 

 tension of which is regulated by a screw. The field -is produced by 

 a horse-shoe magnet, with branches very close, and a hollow cylinder 

 of soft iron is placed inside the coil. This cylinder is magnetised 

 almost as a conductor would be electrified in an electrical field of the 

 same shape (387). The deflections are read by means of a mirror. 



862. LIPPMANN'S GALVANOMETER. With the same order of 

 ideas we may associate the galvanometer of M. Lippmann.* Two 

 vertical tubes containing mercury (Fig. 166) are connected at the 



Fig. 1 66. 



bottom with a very narrow cavity comprised between two parallel 

 glass plates, and placed in a magnetic field, the direction of which is 

 at right angles to the plane of the plate. 



The current conveyed by two platinum plates passes vertically 

 through the liquid contained in this cavity. If the cavity is rect- 

 angular, and of height a, the action of the field H on the current 

 is equal (458) to IH#, and tends to displace the liquid in a certain 

 direction ; a difference of level is produced between the two branches 

 of the mercury. If p is the difference of the corresponding pressure, 

 and e the thickness of the cavity, the condition of equilibrium of the 

 liquid is 



* LIPPMAXN. Comptes rcndus, Vol. xcvm., p. 1256. 1884. 

 R2 



