110 On a Method of exhibiting fine Galvanometric Experiments. 



such as the experiments with the nerve current, and the negative 

 variation of the muscular current in the hving human body, to a 

 numerous audience in the theatre of the Royal Institution. 



The means made use of readily suggest themselves. They 

 consist in furnishing the magnetic system with a reflecting sur- 

 face, by which a bundle of parallel rays are reflected and thrown 

 upon a screen, the motions of the luminous image being observed 

 instead of those of the needle itself. When the light is sufl[i- 

 ciently intense, the moving image can of course be shown to any 

 number of hearers, and it is also manifest that an almost un- 

 limited sensibility is attainable by this method. Helmholtz and 

 I had already conversed on this subject ; indeed he informed me 

 by letter, that he had successfully applied the method several 

 years ago in repeating my experiments. He made use of a gal- 

 vanometer of tangents devised by himself, which is essentially 

 the same as that since empirically found by Gaugain, and the 

 principle of which has been developed by Bravais. A reflecting 

 steel disc supplies, according to Weber's suggestion, the place of 

 the bar -magnet and mirror originally suggested by Poggendorfi'. 

 Making use of sunlight, Helmholtz thus succeeded in showing 

 the principal phsenomena of the muscular current. 



I had placed at my disposal a multiplying galvanometer of 

 28,780 coils, constructed by Sauerwald of Berlin, according to 

 my directions, for my friend Dr. Bence Jones. For this instru- 

 ment I obtained from the same artist an astatic system of thicker 

 needles than ordinary, united together by a piece of brass, instead 

 of the tortoise-shell which I generally use. A prolongation of 

 the uniting piece above the upper needle carries an extremely 

 light ring of brass, at the upper point of which is the eye made 

 use of to suspend the system. Within the ring moves upon a 

 horizontal axis a mirror, fixed in a brass setting of extreme light- 

 ness. The mirror, which I owe to the kindness of M. Schieck, 

 consists of a small round glass cover, made use of for microscopic 

 pui'poses, silvered at one side, and possessing a diameter of 19'5 

 millims. Two simple cocoon fibres are sufiicient to sustain the 

 whole securely. The ring, in which the mirror may be inclined 

 towards the horizon, can itself be turned round the prolongation 

 of the brass joining piece; so that it is thus in our power, by 

 propei'ly deflecting the system, to reflect the incident beam of 

 light in any required direction. It is, however, to be remarked, 

 that when the inclination of the mirror towards the horizon 

 augments, the sensibility of the arrangement diminishes. 



As light-source, an electric lamp from Duboscq in Paris, also 

 belonging to Dr. Bence Jones, was made use of. It was fed by 

 a battery of forty cells of Grove, and our friend Prof. Tyndall 

 had the kindness to undertake the regulation of the light, and 



