and the Maximum of Magnetism of Iron, Steel } and Nickel. 147 



tions were read on a circle graduated to half degrees, and 5 

 inches in diameter. The average diameter of the ring was 16J 

 inches nearly, and was wound with several coils ; so that the 

 sensibility could be increased or diminished at pleasure, and 

 so give the instrument a very wide range. The value of each 

 coil in producing deflection was experimentally determined to 

 within at least £ of 1 per cent, by a method which I shall soon 

 publish. The numbers to multiply the tangent of the deflec- 

 tion by, in order to reduce the current to absolute measure, were 

 as follows : — 



Number of coils. Multiplier. 



1 -05377 



3 -01800 



9 -006007 



27 -002018 



48 .... . -001143 



By this instrument I had the means of measuring currents 

 which varied in strength several hundred times with the same 

 accuracy for a large as for a small current. For greater accu- 

 racy a correction was applied according to the formula of Blan- 

 chet and De la Prevostaye for the length of the needle, the po- 

 sition of the poles being estimated ; this correction in the de- 

 flections used was always less than *6 per cent. To eliminate 

 any error in the position of the zero-point, two readings were 

 always taken with the currents in opposite directions, each one 

 being estimated with considerable accuracy to t j q of a degree. 



The experiments were carried on in the assay laboratory of 

 the Institute, which was not being used at that time; and pre- 

 cautions were taken that the different parts of the apparatus 

 should not interfere with each other. The disposition of the 

 apparatus is represented in Plate II. 



The current from the battery A, of from two to six large 

 Chester's " electropoion " cells No. 2, joined according to ch> 

 cumstances, passed to the commutator B, thence to the tangent- 

 galvanometer C, thence to another commutator D, thence around 

 the magnet E (in this case a ring), and then back through the 

 resistance-coils K to the battery. To measure the magnetism 

 excited in E, a small coil of wire F was placed around it *, which 

 connected with the galvanometer H, so that, when the magnet- 

 ism was reversed by the commutator D, the current induced in 

 the coil F, due to twice cutting the lines of force of the ring, 

 produced a sudden swing of the needle of H. As the needle swung 

 very freely and would not of itself come to rest in ten or fifteen 

 minutes, the little apparatus I was added : this consisted of 



* If a bar was used, this coil was placed at its centre. 



