210 G. F. Barker— New Vertical-lantern Galvanometer. 
its lower end.* Surrounding this lower needle is a circular 
coil of wire, having a cylindrical hollow core an inch in 
diameter, in which the needle swings, and a smaller opening 
transverse to this, through which the suspension wire passes. 
In the apparatus already constructed (in which the upper needle 
is five centimeters long) the coil is composed of 100 feet of No. 
14 copper wire, and has a resistance of 0°235 ohm. The 
accompanying cross section (fig. 2) of the 
vertical-lantern galvanometer as at present 
arranged, drawn on a scale of jz, will serve 
to make the above description more clear. 
A is the needle, suspended directly above 
the seale-glass D, by a silk filament passing 
through the loop B, close under the objec- 
tive C. This needle is attached to the alu- 
minum wire ab, which passes directly 
through the scale-glass D, the condensing 
E, and the inclined mirror F, at H, and 
carries, near its lower end, the second nee- 
dle I. This needle is shorter (its length 18 
2°2.cm.) and heavier than the upper one, 
and moves in the core of the circular coil J, 
whose ends connect with the screw-cups at 
This coil rests on the base of the laa- 
enclosed in a suitable frame. It 1s 
course determined by the distance of the galvanometer from 
the screen; in class experiments, a circle eight feet in diameter 
is sufficient; though in the lecture above referred to, the circle 
was sixteen feet across, and the needle was fourteen feet long, 
the field being brilliant. : 
e method of construction which has now been described, 
* After the new galvanometer was completed and had been in use for several 
weeks, I observed, in re-reading Mayer's first paper, a note stating that the idea 
had 1 to him of using an astatic combination consisting of two needles, 
2 1 inclined mirror, two being conn 
by a stiff wire passing through holes in the condenser and the mirror. The 
at nilanine sha May Bart, eee “I ree | + tn hawn D | 
to him. Indeed, it does not appear that the arrangement he mentions was ever 
carried into practical effect. : ; 
