170 Prof. E. Edlund on the Construction of 



meter for electrical discharges cannot well be provided with an 

 astatic system : a single needle must be used; and to make it 

 more delicate, either a portion of the directive force of the earth's 

 magnetism must be compensated by external magnets, or, what is 

 better, a mirror with telescope and scale may be used. If in using 

 a single needle the suspending thread had no tendency to tor- 

 sion, the delicacy of the instrument would be independent of the 

 strength of the magnetism in the needle; for the directive force 

 of the needle in this case would increase or decrease in the same 

 ratio as the action of the current upon it. The position of equi- 

 librium of the needle would also be independent of the strength 

 of the magnet, provided the position of the magnetic axis in the 

 needle were unchanged. The position of equilibrium is also un- 

 changed if the strength is increased or diminished, provided the 

 force of torsion of the thread only tends to bring the needle into 

 the magnetic meridian. Hence, in order that the instrument 

 may retain as far as possible its delicacy, and moreover not have 

 its position of equilibrium altered by changes in the strength of 

 the magnetism which powerful electrical discharges may cause, the 

 directive force which the suspending thread exerts on the needle 

 in virtue of its torsion must be small as compared with the action 

 of the earth's magnetism, and the position of equilibrium caused 

 b) r torsion must coincide with the magnetic meridian. Accord- 

 ing to Professor lliess*, the magnetism of the needle is greatly 

 protected if between it and the coils there is a thick copper 

 sheath, which at the same time acts as a damper in bringing the 

 oscillating needle quickly to rest. 



But it is easy to see that the action of the electricity on the 

 magnet is not the sole or even the principal cause of the change in 

 the position of equilibrium which results from the passage of the 

 electrical discharge through the coils of the galvanometer. The 

 galvanometer which I used in my former experiments on the 

 electromotive force of the electrical spark had a single needle, 

 which was firmly connected with a mirror, by the aid of which 

 the deflections were read off by the telescope and scale in the 

 ordinary manner. The mirror consisted of glass, and the back 

 was covered with a thin metal disk. The galvanometer-wire, 

 which consisted of copper, was 1 millim. in diameter, and was 

 surrounded by a coating of gutta percha 2 millims. thick. Hence 

 the entire thickness of the wire, including the insulating coating, 

 was 5 millims. This wire was wound in forty coils round a ma- 

 hogany frame. The aperture in the frame, in which the magnetic 

 needle was suspended by a cocoon-thread, was 50 millims. in 

 length by 30 in height. The length of the needle was 42 mil- 



* Abhandlung : Ci Zu der Lehre von der Reibungs-Electricitat," Berlin, 

 1867, p. 3\4. 



