( 648 ) 



disadvantageous five of tliose elements, in order to obtain an 

 equal distribution of the current each with a series-resistance of 

 5000 ^i, are connected parallel to each other. Finally, when the 

 adjustment is obtained, the battery which has served in the regulation 

 is replaced by altering a plug in the commutator, by a battery 

 arranged in the same way, which serves for the measurements. 



The peculiarit}' of this arrangement is that we can make imme- 

 diately after each other all the readings required for the measurements 

 by handling the commutator-boards (comp. successively PI. V Comm. 

 N". 27) and without making other contacts than those of mercury- ; this 

 would be impossible without the commutators and the current-rever- 

 sers with mercury -contacts. This again reveals the great advantages 

 which these apparatus offer for similar measurements. It seems, 

 however, that little attention has been paid to them. 



The whole arrangement with 6 current-rexersers 1, 2, 3, 4, 5, 6 

 and 4 commutators A, B, C, D for both deflection- and zero-method 

 as it has become now may be easily seen on PI. II. The current- 

 reversers and commutators with mercury-contacts have been indicated 

 by three and four parallel lines respectively. 



1 serves to connect one of the two galvanometers, that of Hart- 

 MAXN and Brain, described in Comm. K". 27 or a magnetically 

 protected galvanometer of Du Bois and Rubens. 



2 and 4 to switch the galvanometer on to one of the three 

 branches A, B, C, 



3 to switch the battery of comparison-elements on to the branch 

 A or on to B, 



5 to introduce the observation-element and the comparison-battery 

 separately or in series or in opposition (cf. 3), 



6 to introduce either the observation-element or the comparison- 

 elements. 



A, B and C make and reverse the connection of the three circuits 

 with the galvanometer, D reverses the accunudator. The commutators 

 and tlie current-reA'ersers, like the Weston- battery have Iteen packed 

 in cottonwool, placed together in large cases. Only the tubes where 

 the contact of the mercury is made by handling tlie commutator- 

 board (see PI. V Comm. N". 27) project beyond it. 



