Dr. A. M. Mayer on a new Lantern-Galvanometer. 27 



coated with paraffine, and are wrapped on the faces of vulcanite 

 disks. The spirals have an internal diameter of 4 inches and 

 an external diameter of 10 inches ; and each contains 49 feet of 

 wire in 26 turns. The four terminals of the spirals are con- 

 necting-screws, two of which serve to join the spirals so that a 

 current will circulate in the same direction in both. The spirals 

 are so placed that a line joining their centres will pass through 

 the centre of the magnetic needle. 



The vertical lantern rests on a base SJ feet long, with guides 

 on its sides, between which slide boards carrying two bar-mag- 

 nets, A and B, 15 inches long and 1 inch in diameter, as shown 

 in the figure. These magnets can not only approach to and re- 



cede from the lantern, and thus alter their distances from the gal- 

 vanometer-needle, but they can also rotate around their centres on 

 vertical axes. The like poles of the magnets and of the needle 

 point in the same direction ; and by sliding the magnets to or 

 from the lantern -needle we render the latter more or less astatic. 

 Also, in case the needle should not hold to the meridian as you 

 approach the magnets, it can be made to do so by rotating one or 

 both of them in the horizontal plane ; and thus also can be neu- 



