Dr. A. M, Mayer on a new Lantern-Galvanometer, 29 



Exp. 4. — A coil of 20 inches diameter, having five turns of 

 g'o-inch wire, was rotated 180° round a vertical axis at a dis- 

 tance of 3J feet from end of magnet. Deflection of needle 

 was 30°. 



Exp. 5. — Same as Exp. 4, only coil had ten turns of wire in- 

 stead of five. Galvanometer-needle deflected 50° to 60°. 



Ejcp. 6. — A coil of 20 inches diameter, formed of ten turns 

 of 2V-iiich wire, was revolved 180° round a vertical axis 6J feet 

 from end of magnet. Deflection of needle 22°. 



Exp. 7. — The coil used in Exp. 6 was placed 3 feet 8 inches 

 above centre of axis of the magnet, and revolved 180° round a 

 vertical axis ; the needle was deflected 80°. 



Exp. 8. — A coil of 2\ feet diameter, formed of forty turns of 

 300 feet of j^Q-iuch wire, was placed 28 feet distant from the centre 

 of the magnet, and with its plane coinciding with the plane of 

 the magnetos equator. On rotating it round a vertical axis the 

 needle was deflected 20°. 



The following experiments will show the excellent proportions 

 (arrived at by a long series of experiments) of the coil used in 

 Exps. 1, 2, and 8, for the evolution and study of the electric 

 currents induced by the earth's magnetism. 



Exp. 9. — The coil used in Exps. 1, 2, and 8 was laid on a 

 table, and its terminals connected with a galvanometer which is 

 used in connexion with Nobili's thermo-electric pile. The 

 needles of this instrument made one oscillation in nine seconds. 

 I lifted the east side of the coil only six inches ; the needle was 

 deflected 10°. Lifting the same side nine inches, the needles 

 went to 22°. I now placed the coil in a north and south vertical 

 plane ; and suddenly tilting its top six inches to the east or to 

 the west, the needles went to 60°. Tilting the coil nine inches 

 sent the needles with a blow against the stop at 90°. 



The advantages of the new galvanometer may be summed up 

 in a few words. It gives on the screen a bright clear image of 

 only the graduated circle and of the needle. It can readily be 

 rendered more or less astatic to adapt it to the character of the 

 electric currents worked with. The direction of its needle is 

 completely under the control of the damping-magnets; and, 

 finally, it is of simple construction, and can be rapidly adjusted 

 to the requirements of any special experiment. 



