54 Mr. J. Parnell on Secondary Currents 



When the commutator is turned off, by depressing the key Z 

 the Daniell current is sent through the resistance-coils to the 

 galvanometer. 



4. The modus operandi will now be quite apparent : the index 

 having been carefully adjusted to zero, the commutator was 

 turned so as to excite the secondary couple, and at the expiration 

 of twenty seconds, the interval having been taken by the ticks 

 of a clock beating dead seconds, the Morse key was depressed, 

 the index-reading observed, the commutator turned off, and a 

 short circuit made to exhaust the secondary couple before another 

 experiment was made. This process was repeated in each case 

 till five results had been obtained, the mean of which was recorded. 

 After the first, third, and fifth experiment the deflection of the 

 galvanometer by the Daniell cell was observed and the mean of 

 these results noted. The ratio of the secondary to the Daniell 

 mean, multiplied by 10 3 , is the figure of merit recorded in this 

 paper, and gives the value of the couple under examination, 

 that of the Daniell cell being 1000. During a set of five conse- 

 cutive experiments, the index-reading due to the Daniell cell 

 never differed from the mean by more than one division of the 

 scale. The following instance of a set of experiments, taken at 

 random from the many hundreds which have been made on this 

 subject, will give a fair notion of the results obtained (Table I.) : — 



Table I. 



No. of 



Secondary 



Daniell 



Ratio of means, or 



experiment. 



readings. 



readings. 



figure of merit. 



1. . . 



. 157 



176 





2. . . 



. 158 







3. . . 



. 157 



176 





4. . . 



. 158 







5. . . 



. 157 



175 





Means . . 157*4 175-6 896 



Unless otherwise mentioned, throughout the whole course of 

 the experiments herein described 9 per cent, of the currents was 

 shunted from the galvanometer ; and the resistance employed 

 was 7000 B. A. units, in comparison with which all the other ex- 

 ternal resistance, and a fortiori any accidental variations in it, 

 vanished. 



5. Some preliminary experiments showed that copper plates 

 in a solution of carbonate of sodium formed a powerful secondary 

 combination ; and, to see what advantage it possessed relatively 

 to known secondary arrangements, it was compared with plati- 

 num, palladium, and lead in dilute sulphuric acid (1 part acid, 

 6 parts water). The specific gravity of the solution of the car- 

 bonate, as well as of all the other alkaline solutions mentioned 



