Determination of the Ohm. 421 



The galvanometer was not artificially damped. The posi- 

 tion of rest of the needle was determined from its maximum 

 divergence. The divergences to be observed, ol x and « 2 ? were 

 never less than 800 millim. on a scale 4 metres distant, and 

 were always within a few scale-divisions of each other; so that 

 for the reduction to arcs an approximate knowledge of the 

 distance of the scale was enough. All connecting- w T ires were 

 covered with caoutchouc and twisted lightly together; all the 

 current-reversers, &c. w r ere made of paraffin and sealing-wax. 



Altogether, sixty-seven experiments were made ; only the 

 . solenoid is common to them all. 



Here the only variation possible was that the induction- 

 bobbin could be placed 10 centim. further towards either end, 

 instead of in the middle. All other components of the 

 apparatus were altered in each experiment to a considerable 

 extent. The sections of the induction-coil were taken either 

 separately or in combinations of 2-5, corresponding to which 

 the number of the intermittences was 5-13 per second. The 

 strength of the inducing current was from 0*0005 to 0'01 

 ampere. The sensitiveness of the galvanometer was con- 

 siderably alterable. The time taken by the oscillations of 

 the needle was from 15 to 34 seconds. 



To ascertain if the induction-current passing through the 

 galvanometer produced any other cross-magnetization of the 

 needle, besides constant currents of otherwise equal galvano- 



metric action, bv which the relation would be rendered 



7 J tan oti 



unreliable, several experiments were made with magnets of 

 a diameter of 0*6 centim., those used in the above-mentioned 

 experiments being hardly 0'1 in diameter. No difference 

 was observed after a certain number (43) of experiments had 

 been made ; the interruptor was fitted up afresh, and all con- 

 necting-wires as well as current-reversers renewed. 



As the following results tally with the former, it may be 

 assumed that the isolation was perfect. 



The whole plan of experimenting is based on the constancy 

 of the source of electricity, be this periodic, as in the observa- 

 tions by means of induction-currents, or continuous, as in the 

 measurement by means of the constant current. Means had 

 therefore to be taken to meet the final objection that errors 

 might arise through possible polarization in the battery. 

 With this object, I used as sources of the current: — 



1. 1 to 4 DanielPs elements. 



2. Two points of a w T ire-resistance connecting a number of 



Daniell's or Bunsen's elements. 



3. A thermo-element. 



Phil. Mag. S. 5. Vol. 20. No. 126. Nov. 1885. 2 H 



