416 A. M. Mayer—New form of Lantern-Galvanometer, 
To deflect this needle by means of an electric current I place 
as close to the condensing lens as possible the two vertical wire 
spirals 8, S, formed of 4; inch copper wire of square section, so 
as to bring the convolutions as close together as possible. The 
turns of the spirals are separated with very thin vulcanite rib- 
bon, coated with paraffine, and are wrapped on the faces of vul- 
ite di The spirals have an internal diameter of four 
inches and an external diameter of ten inches, and each contains 
49 feet of wire in 26 turns. The four terminals of the spirals 
are connecting screws, two of which serve to connect the spirals 
so that a current will circulate in the same direction in both, 
The spirals are so placed that a line joining their centers will 
pass through the center of the magnetic needle. 
The vertical-lantern rests on a base three feet and a half 
long, with guides on its sides, between which slide boards 
carrying two bar-magnets, A and B, 15 inches long and one 
inch in diameter, as shown in the figure. These magnets can- 
not only aes to and recede from the lantern ‘in 
ke poes 
% 
alter their distances from the galvanometer-needle, but 
so rotate around their centers on vertical axes. The % 
of the magnets and of the needle point in the same pen a 
and by sliding the magnets to or from the lantern-D 
