38 KEPORT— 1876. 



Wheatstone's bridge, the condition of equilibrium for the galvanometer will 

 be different for weak currents and for strong ones. But since a strong current 

 heats the fine wire much more than the thick wire, the law of Ohm could not 

 be tested by any ordinary observation, first with a weak current and then 

 with a strong one, for before the galvanometer could give an indication the 

 thin wire would be heated to an unknown extent. 



In the experiment, therefore, the weak and the strong current were made 

 to alternate 30 and sometimes 60 times in a second, so that the temperature 

 of the wire coiUd not sensibly alter during the interval between one current 

 and the next. 



If the galvanometer was observed to be in equilibrium, then, if Ohm's law 

 is true, this must be because no current passes through the galvanometer, 

 derived either from the strong current or the weak one. But if Ohm's law is 

 not true, the apparent equilibrium of the galvanometer-needle must arise from 

 a succession of alternate currents through its coil, these being in one direction 

 when the strong current is flowing, and in the opposite direction when the 

 weak current is flowing. 



To ascertain whether this is the case, wo have only to reverse the direction 

 of the Aveak current. This will cause the alternate currents through the gal- 

 vanometer-coil to flow both in the same direction, and the galvanometer will 

 be deflected if Ohm's law is not true. 



Mr. Chrystal has drawn up a report of this second experiment, giving an 

 account of the mode in which the various difficulties were surmounted. 

 Currents were employed which were sometimes so powerful as to heat the 

 fine wire to redness ; but though the difficulty of obtaining a steady action of 

 the apparatus was much greater with these intense currents, no evidence of 

 a deviation from Ohm's law was obtained ; for in cverj- experiment in which 

 the action was steady, the reversal of the weaker current gave no result. 



The methods of estimating the absolute values of tho currents are described 

 in the Keport. 



A third form of experiment, in which an induction-coil was employed, is 

 also described ; but though this experiment led to some very interesting re- 

 sults, the second experiment gives the most searching tost of the accuracy of 

 Ohm's law. Mr. Chrystal has put his result in the following form. 



If a conductor of iron, platiniim, or German silver of one square 'centimetre 

 in section has a resistance of one ohm for infinitely small currents, its re- 

 sistance when acted on by an electromotive force of one volt (provided its 



temperature is kept the same) is not altered by so much as -^tj^ part. 



It is seldom, if ever, that so searching a test has been applied to a law which 

 was originally established by experiment, and which must still be considered 

 a purely empirical law, as it has not hitherto been deduced from the funda- 

 mental principles of dynamics. But the mode in which it has borne this test 

 not only warrants our entire reliance on its accuracy within the limit of 

 ordinary experimental work, but encourages us to believe that the siinplicity 

 of an empirical law may be an argument for its exactness, even when we are 

 not able to show that the law is a consequence of elementary dynamical 

 principles. 



First Experiment. Christmas 1875. By Gr. Chrystal, Cavendish Laboratory, 

 Cambridge. Communicated by J. Cleek Maxwell. 

 If the electromotive force between two points of a uniform linear con- 

 ductor measured in appropriate units by means of an electrometer be E, and 



