employed for determining the Ohm. 271 



duction-coils are so close together, a small error in measuring 

 their distance apart is of great influence — the more so since, 

 from what has been said, it follows that the position of the 

 separate coils could not be accurately controlled. That the 

 distance of the coils apart at various points was measured to 

 the ^o millim., and that the mean distance apart is given to 

 Yoos millim., and the mean radius to the joooo millim., can 

 hardly be regarded; in view of the above-mentioned disturbing 

 causes, as a guarantee of the actual exactitude of the results. 



Since the length of the needle of the tangent-galvanometer 

 (2*7 centim.) amounted to only -^ of the diameter of the coil 

 (50 centim.), the deviation from the law of tangents is imper- 

 ceptible ; but, on the other hand, there is an important source 

 of error in the fact that the needle turned upon a point. 

 Rowland asserts that the needle, which was provided with a 

 pointer playing over a circle of 20 centim. diameter, always 

 took up its position accurately to 1 or 2 minutes (1 minute 

 corresponds to only 0*03 millim. upon the circle). Since this 

 does not at all agree with previous experience, an exact ex- 

 planation how so great sensitiveness was obtained is indis- 

 pensably necessary. 



The needle of the galvanometer was 1'25 centim. long ; 

 the coils, which were brought up to it -from the east and west, 

 had an internal diameter of 3 centim. and an outer diameter 

 of 5*6 centim. ; their inner end-surfaces were at a distance of 

 0*9355(35 centim. (thus given to xooWo millim.) from the 

 point on which the needle turned. Whether the law of tan- 

 gents is altogether applicable here must remain uncertain. 



Since the intensity of the inducing and induced currents is 

 measured at different places, Rowland introduces a further 

 complication by determining the ratio of the horizontal com- 

 ponent of the earth's magnetism at the position of the tangent- 

 circle and of the galvanometer by surrounding the galvano- 

 meter with a larger coil, sending the same current through it 

 and the tangent-circle, and comparing the deflections. The 

 radius of the new circle (some 4 centim.) is again given, to 

 Toko millim. 



The values of the different dimensions given to so many 

 decimal places are probably not regarded by the author him- 

 self as accurate in the same degree, the last decimals (obtained 

 by interpolation) being added for the sake of completeness. 

 It is, however, always advisable to give exact account how far 

 actual observation goes, since the mean values can never be 

 accurate beyond the limit of possible observation, or else with 

 the roughest measures it would be possible by repeating mea- 

 surements to obtain any required degree of minuteness. 



