Vertical- Lantern Galvanometer* 



437 



beneath the object-glass, and thence down to the side pillar 

 which supports this objective, where it is fastened by a bit of 

 wax, to facilitate adjustment. The needle itself is fixed to an 

 aluminium wire, which passes down through openings drilled 

 in the scale-glass, the horizontal lens, and the inclined mirror 

 and which carries a second needle near its lower end*. Sur- 

 rounding 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 centimetres 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. 



(fig. 2) of the vertical-lantern galvanometer 

 as at present arranged, drawn on a scale 

 of T V, will serve to make the above de- 

 scription more clear. A is the needle, 

 suspended directly above the scale-glass D, 

 by a silk filament, passing through the 

 loop B, close under the objective C. This 

 needle is attached to the aluminium wire 

 a b, which passes directly through the 

 scale-glass D, the condensing lens E, and 

 the inclined mirror F at H, and carries, 

 near its lower end, the second needle I. 

 This needle is shorter (its length is 2*2 

 centimetres) and heavier than the upper 

 one, and moves in the core of the circular 

 coil J, whose ends connect with the screw- 

 cups at K. This coil rests on the base of 

 the lantern, enclosed in a suitable frame. It is obvious that, 

 when the instrument is so placed that the coil is in the plane of 

 the magnetic meridian, any current passing through this coil 

 will act on the lower needle ; and since both needles are attached 

 to the same wire, both will be simultaneously and equally de- 

 flected. Upon the screen is seen only the graduated circle and 

 the upper needle ; all the other parts of the apparatus are 

 either out of the field or out of focus. Moreover the hole in 



* 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 occurred to him of using an astatic combination 

 consisting of two needles, one above the lens and the other below the 

 inclined mirror — the two being connected by a stiff wire passing through 

 holes in the condenser and the mirror. The plan of placing the coil 

 round the lower needle does not seem to have suggested itself to him. 

 Indeed it does not appear that the arrangement he mentions was ever 

 carried into practical effect. 



