14 REPORT — 1866. 



red curves were made to represent the whole effects of the perturbations, or the 

 quantities which it is necessary to apply to the line of no distiu-bance, reckoned a 

 straight line, in order to reconstruct the ciirves with the perturbations. 



Thus the effect of disturbances upon the declination is to cause the needle to 

 deviate towards the west diu-ing- the hours of the day, but towards the east during 

 the hours of the night. 



The effect of disturbances upon the vertical force is of a reverse kind, tending to 

 diminish this element during the hours of the day, but to increase it duiing those 

 of the night. 



With regard to the horizontal force, it appears that the distm-bances tend to 

 diminish this element almost diu-ing the whole of the twenty -four hom-s. 



A third diagram represented the mean hom-ly movements of the north pole of 

 the freely suspended needle in a plane perpendicular to the direction of such a 

 needle, both for the whole year, and also for the winter and simmier seasons. 



On certain Phenomena winch presented themselves in Conne.vion with the 

 Atlantic Calle. Bi/ C. F. Yaklet. 



On a Neiu Method of Testing Electric liesistance. Bi/ C. F. Taeley. 

 In 1860, Prof. Thomson and Fleemiug Jenlrin, F.R.S., invented a method of ob- 

 taining exact subdivisions of the potential of a voltaic battery. The apparatus con- 

 sisted of a number of equal resistance-coils, sa}' 100. These were connected one 

 witli one pole of the battery, and the other with the other pole. To the function 

 of each coil a piece of metal is attached, and a spring attached to a brass slide tra- 

 velling along a square rod of the same metal traverses these different pieces, and so 

 makes contact with whichever is desired. If the two poles of an electrometer be 

 attached, the one to one pole of the battery, and the other to the brass bar on which 

 the slide travels, it will be found that at the one end we have the full potential 

 power, and at the other end nothing at all, and halfway half the potential; this is 

 too self-e\-ident to require furflier explanation, and is explained in Thomson and 

 Jeukin's patent, 1860. Prof. Thomson has recently succeeded in making reflecting 

 electrometers of such sensiWlity that they will give 200 scale-divisions for a varia- 

 tion of potential equal to one cell of Danieirs battery. In testing the Atlantic 

 Cable this electrometer was used in the following way at Yalentia, to get the po- 

 tential of the ship's magnetism. The one pole of the electrometer was connected 

 with the cable, and the other one with the slide, and by running it up and down 

 the exact potential of the cable was measured. There were in the jnaia slide ICO 

 coils of 1000 units each, and it became necessary to subdivide these again 100 

 times to get sufficient accuracy. Some difficulty presented itself in getting a 

 method for subdividing these coils, and the author v^as fortunate enough to hit 

 upon the following verj' simple method of effecting this pm-pose. The slide con- 

 sists of two square brass bars, over each of which travels a piece of brass, to the 

 bottom of which is attached a spring, pressing upon the studs connected ■«'ith 

 the resistance-coils. Instead of using 100 coils in the main slide, the author 

 uses 101, and makes the two springs to embrace two coils. Thus, then, the 

 two bars of the slide have invariabh' a resistance between them of 2000 ohmads. 

 The two bars are connected with a second set of 100 coils, each coil hav- 

 ing 20 units resistance, and the 100 coils making up precisely the same resistance 

 as that of two of the coils in the main slide. These two circuits of 2000 units each 

 reduce ihe resistance to one-half, or to 1000 units, so that the resistance of the 101 

 coils of 1000 each is reduced to that of 100 coils. By passing the traveller along 

 the 20 unit coils in the second slide an exact subdivision of the potential between 

 these points is obtained ; and in this way the potential of the battery is accurately 

 and quickly subdivided to 10,000 parts. By these means Prof. Thomson has beeii 

 able to introduce a method of testing, on the Wheatstoue balance system, so ex- 

 tremely simple that it should be made known as soon as possible. 'The battery is 

 connected permanently to the main slide, so that a current is always passing through 

 it. Its resistance, 100,000 ohmads, is such that no sensible elevation of tempera- 

 ture is produced. The cm-rent is also passed into the cable through a definite re- 



