16 REPORT 1805. 



introduced in the current, all the nineteen striae assume the double-convex form, 

 the blue being far more intense or vivid, particularly in those nearest the negative, 

 the red line in the centre continuing in all. In this state of the discharge the 

 tube was placed between the poles of a very powerful electro-magnet ; and on 

 exciting the magnet the disks separated, precisely as if a resistance had been intro- 

 duced, but deflected to the upper or lower part of the tube, according to the 

 direction of the magnetic power. On the sides of the tube, where four or five of 

 the disks nearest the negative impinge, there at present remains a black deposit 

 similar to that deposited from the negative metallic wires by an induction-coil. 

 As much heat is evolved at the negative wire, and until the author obtains an- 

 other tube which will exhibit the same results be does not like to risk its destruc- 

 tion, he is thus prevented from continuing the experiment to the extent he desires : 

 but if it is found that a deposit can be obtained at defined portions of the 

 stratified discharge, it may probably assist in explaining phenomena which have 

 hitherto baffled the researches of every electrician. 



On a New Method, introduced by Messrs. Siemens, for the Measurement of 

 Electrical Resistances. By 11. Sabine. 



The insulation of submarine-telegraph cables was, until the Red-Sea and Indian 

 lines were submerged, in 1859, determined qualitatively by the simple deflection of a 

 galvanometer-needle. The manufacture of the core of the Malta cable, with the 

 scrupulous surveillance of its electrical conditions, formed an era in cable-work ; 

 and Messrs. Siemens, who were intrusted by the Government with the duties of 

 electricians to the cable, may be said to have then first established with success the 

 science of cable-testing. After various improved plans, the Messrs. Siemens intro- 

 duced, in 1860, for cable-work, a differential galvanometer with two coils, one of 

 which exerted two thousand times as much deflective force upon the needle as the 

 other. The cable and the measuring-battery were inserted in the circuit of the 

 larger helix, and a single element with a set of resistance-coils in that of the less 

 sensitive helix of the instrument. The resistance in the latter circuit was altered 

 until the magnetic forces of the two currents upon the needle were equal and oppo- 

 site and the pointer rested over the zero-line of the card. The manipulation was 

 simple, and the necessary calculations reduced to the mere multiplication of the 

 value of the resistance in the smaller circuit by the constant of sensibility and the 

 relation of the electromotive forces. On the fitting out of the Carthagena cable 

 it was considered desirable in measurements of insulation to dispense with mathe- 

 matical reductions, and to read off the resistances directly from the instrument. It 

 is easily understood that, to establish an equilibrium between the magnetic forces 

 of two coils of a differential galvanometer, it is not absolutely necessary to alter 

 the currents in either of the circuits ; the same may be attained by altering the re- 

 lative distances of the coils from the needle. This is the principle upon which the 

 new differential galvanometer, forming the subject of this communication, was 

 based. The mechanical construction of the instrument is very simple. A pair of 

 astatic needles are suspended by a fibre of unspun silk between about 10,000 turns 

 of a long, thin, well-insulated copper wire, their position being indicated by an 

 aluminium poiuter fixed across their axis, moving over a dial-card and observed 

 through a magnifying-glass. Outside the case of the instrument is a horizontal 

 metal stage, upon which a vertical coil of insulated copper wire is moved to and 

 from the instrument by means of a micrometer screw. The theory of the method 

 of this instrument and its use in testing cable resistances is this : — Two galvanic 

 currents circulate in the two coils — the stationary and the moveable — in oppo- 

 site directions, and will oppose each other in their effects upon the needle, which 

 will take up a position at an angle less than that which it would if the stronger 

 coil were alone active. By altering the position of one of the coils a point is 

 reached where the deflective force of one coil is made to exactly counterbalance 

 the force of the other, and the needle returns to zero. To measure the resistance 

 of a cable with this instrument, then, nothing more is necessary than to put it in 

 the circuit of the larger coil aud to vary, by means of the micrometer-screw, the 

 position of the moveable coil until equilibrium is obtained. The distance is then 



