910 
Journal of Agricultural Research 
Vol. XXIV, No. It 
Table II. Time^ required for electrodes to record maximum difference in potential; 
hydrogen passed over electrodes rapidly {readings recorded as millivolts) 
74- 
74- 
75- 
75-' 
76. 
76. 
24. 
24.. 
26.. 
26. . 
28.. 
28.. 
209. 
209. 
210. 
210. 
191. 
191. 
192. 
192. 
261;. 
265. 
166. 
166. 
38.. 
38.. 
8 . .. 
8 .. . 
Soil No. 
Time (in minutes). 
678 
668 
674 
670 
583 
59° 
648 
630 
662 
665 
690 
682 
576 
535 
540 
536 
575 
565 
556 
525 
548 
548 
688 
681 
683 
685 
587 
592 
574 
570 
a 546 
546 
IS 
« 691 
680 
a 688 
688 
586 
591 
a 675 
® 667 
a 670 
a 670 
690 
680 
573 
“577 
546 
“ 35° 
a 562 
364 
® i ;62 
360 
“ 313 
“ 313 
690 
^ 682 
688 
690 
585 
590 
25 
588 
586 
563 
557 
582 
577 
690 
682 
688 
692 
584 
588 
674 
664 
666 
670 
686 
688 
562 
a 566 
560 
a 564 
515 
515 
30 
514 
514 
586 
584 
562 
561 
585 
584 
35 
360 
364 
338 
364 
362 
360 
385 
“385 
a Maximum reading. 
The platinum electrodes used were coated with platinum black and 
tested on a standard acetate solution before using. Several determina¬ 
tions could usually be run with one coating of platinum black. Dupli¬ 
cate samples of soil were always run and as a rule the results agreed 
within 10 millivolts. If the disagreement were much greater than this, 
the sample was again run. Sharp and Hoagland (13) state that “ Dupli¬ 
cate determinations on soil suspension usually agreed within 0.01 to 0.02 
volt.'’ Plummer (ii) says ‘'Duplicate readings on the same sample 
of soil could easily be read to 0.02 volt, ” while “ It was almost impossible 
to get such closely agreeing results as 0.02 volt with different samples 
of the same soil.” T^he data presented in Tables I, II, III, and IV 
illustrate the millivolt readings of duplicate samples run on different 
electrodes. In Table IV are shown the millivolt readings of samples of 
the same soil run on different dates and also the slight effect upon the 
reading of varying the ratio of soil to water. 
