28 Dr. A. M. Mayer on a new Lantem- Galvanometer. 



tralized any exterior disturbance which may tend to deflect the 

 needle from the magnetic meridian. 



The needle may be also rendered astatic in the usual way by 

 suspending it by a silk fibre, and attaching to this needle a wire 

 which passes through a hole in the condenser and in the inclined 

 mirror and carries beneath the latter another needle with poles 

 reversed. 



In working with thermal currents we use a smaller needle 

 and condenser, which allows the spirals to approach nearer j but 

 for thermal currents it is better to wind close round the needle 

 a flat coil only one wire in breadth, and to use a suspended 

 astatic system, of which the lower needle is the stronger and is 

 under the control of the damping-magnets*. The breadth of 

 the coil used in this last device need not exceed -^^ of an inch ; 

 and its image on the screen can answer for a rough zero-point. 



I will now give a few experiments in which this galvanometer 

 has been employed ; and they will serve to show its usefulness. 



Experiment 1. — A coil of 2^ feet diameter, containing forty 

 turns of 300 feet of -j^Q-inch wire, w^as placed with its plane at 

 right angles to "the dip.^' Its terminals were connected with 

 the galvanometer, whose needle was rendered astatic by means 

 of the damping-magnets. I now quickly rotated the coil 180^ 

 round an axis at right angles to the direction of the dipping- 

 needle. The galvanometer-needle was deflected about 12° by 

 the magneto- electric current induced by the earth^s magnetism. 



Exp. 2. — I placed the coil used in Exp. 1 on a wooden wheel 

 provided with a commutator, and rotated it round an axis at 

 right angles to the dip. The galvanometer-needle went steadily 

 up to a deflection of 85°, and was held there as long as the coil 

 revolved. 



Exp. 3. — The two cores of the large electromagnet of the 

 Stevens Institute of Technology were placed end to end, thus 

 forming one iron bar 7 feet long and 6 inches in diameter. This 

 was surrounded by its eight bobbins, containing in all 2000 feet 

 of ^-inch copper wire ; and through them was sent the electricity 

 developed by the most advantageous combination of sixty plates 

 of zinc and carbon, 10x8 inches. 



A coil of 20 inches diameter, formed of one turn of 2"^ -inch 

 wire, was rotated 180° round a vertical axis 3^ feet from the end 

 of the magnet. The needle was deflected 3°. 



* The upper needle of this astatic combination swings in the interior of 

 the coil which encloses both the needle and the condenser c; the lower 

 needle swings under the inclined mirror M, and is attached to the upper 

 needle by means of a stiff wire, which passes through holes in the condenser 

 and in the inclined mirror. In another combination I have placed this lower 

 needle above the coil, and have " damped " it by means of a magnet placed 

 above the reflector R. 



