1871.] Electricity, 557 



degree of deflection may be read off by a microscope. As yet, we believe, no 

 definite observations have been made with the instrument, which is intended 

 for Kew Observatory. 



MM. de la Rive and E Sarasin have communicated to the Societe de 

 Physique et d'Histoire Naturelle de Geneve a most interesting paper, 

 embodying the results of their experiments on the action of magnetism upon 

 electrified gases. The experiments are detailed at great length, and have 

 already appeared in English in the " Philosophical Magazine." The following 

 are the deductions : — 



" 1. That the action of magnetism when it is exerted only on a portion of an 

 electrical discharge transmitted through a rarefied gas, determines in that por- 

 tion an increase of density. 



" 2. That the same a<5tion, when it is exerted upon an electrical discharge 

 placed eqnatorially between the poles of an electro-magnet, produces in the 

 rarefied gas through which it is propagated an increased resistance, which is as 

 much greater as the gas itself is a good conductor. 



" 3. That this action, on the contrary, determines a diminution in resistance 

 when the discharge is directed axially between the two magnetic poles, 

 this diminution increasing with the conductivity of the gas. 



"4. That when the action of the magnetism consists in impressing a 

 continuous movement of rotation on the electrical discharge, it has no 

 influence on the resistance to conduction, if the rotation is effected in a plane 

 perpendicular to the axis of the magnetised soft iron core which determines 

 the rotation ; while it considerably diminishes it if the rotation takes place 

 so that the electrical discharge describes a cylinder around the axis of the rod. 



" 5. That these different effects apparently cannot be attributed to variations 

 of density produced in the gaseous medium by the magnetic action, but very 

 probably their explanation will be found in the perturbation induced by 

 that action in the arrangement or disposition of the particles of rarefied gas 

 necessary for the propagation of electricity." 



It would appear the decrease of conductivity corresponds to the constrained 

 position into which the electrical discharge is forced under the influence of the 

 magnetism. The gases were inclosed in tubes to which was attached 

 the necessary apparatus for measuring the variation in pressure. 



It is well known that in working the tangent galvanometer with currents of 

 high intensity, the tangents are not proportioned to the strength of the 

 currents, owing to the needle being removed from the direct magnetic field. 

 Professor Trowbridge, of Harvard College, has designed a galvanometer with 

 which the results are more nearly proportioned. It consists of the ring and 

 needle of the ordinary tangent galvanometer, the ring, however, instead 

 of remaining perpendicular, being made to take any angle between the perpen- 

 dicular and the horizontal planes. The deflection of the needle when the coil 

 is horizontal being nil, the different angles of inclination give increased deflec- 

 tions. The deflection remaining constant, the intensities of the currents vary 

 as the cosines of the angles of inclination of the coil. The instrument 

 is therefore termed the cosine galvanometer. Many determinations of the 

 intensity of the same current can be made by forming a table of the values of 

 the cosines of the different angles of inclination in terms of the deflection of 

 the needle which currents with a known interposed resistance produce. In 

 the tangent galvanometer but one determination can be made. The delicacy 

 varies inversely as the cosine of the angle of inclination. With large deflections, 

 therefore, this instrument appears to give closer results than the ordinary tan- 

 gent galvanometer ; and used with a Gaugain's multiplier, very accurate estima- 

 tions can be made. 



