October G, 1905.] 



SCIENCE. 



439 



i? 



E 



E+F+Q+X E+ 



v.hicli readily gives the relation 



E 



It is true this result appears in the same 

 form as that deduced for the Wlaeatstone 

 bridge, but beyond a superficial analogy there 

 is nothing in common between the two meth- 

 ods. The Wheatstone's bridge method con- 

 sists in dividing two parallel circuits in the 

 same ratio. Mance's method, on the other 

 hand, consists in subtracting from the two 

 portions of a single circuit such resistances 

 that the two portions shall still maintain the 

 same ratio to each other. 



In this connection it may be of interest to 

 look at the results of a few measurements by 

 this method. The resistance measured coij.- 

 sisted of a medium-sized storage cell in series 

 with a coil marked ' 2 ohms.' This gives a 

 definite resistance with an E.M.E. not easily 

 polarized. The results of thirty measure- 

 ments are shown in the table below, i? was 

 varied from one ohm to forty ohms, and P 

 ivas given such values that Q would be a little 

 over 4,000 ohms. Each balance was sensitive 

 to a change of 1 ohm in Q, and often the 0.5- 

 ohm coil was used. The results are tabulated 

 in the order obtained, reading across the table 

 from left to right. As the room became 

 warmer the resistance grew larger, each col- 

 iimin showing the same increase of 0.002 ohm. 

 Tt is seen from these results that the method 

 is as sensitive as a post-ofiice box, and by 

 using a larger condenser the sensitiveness can 

 loe still further increased. From this limited 

 data it is hardly safe to draw a general con- 

 clusion, but it may be noted that the smaller 

 "values of R, in other words, the larger cur- 

 Tents in the storage cell, give smaller values 

 of X, the same as with ordinary cells. 



Tempera- 

 ture of 

 Room. 



12. °0 

 12.°6 

 13.°0 



13. °2 

 13. °5 



Resistance of ' 2 Ohms ' Plus Storage Cell. 



iJ = l. lE = 2. I iJ = .3. LB = 



i? = 10. !i? = 40. 



2.0265 !2.02f=>0i 2.0295 2.0295 2.0300; 2.0300 

 2.0265 2.0290 2.0305 2.0310 2.0315 2.0320 

 2.0280 2.0297 2.0315 j 2.0312 2.0315 ' 2.0320 

 2.0282 2.0300 2.031512.0315 2.0317,2.0320 

 12.0285 2.0302, 2.0315 12.0315 2.0317' 2.0320 



The following results were obtained from a 

 large ' Gonda ' cell, a porous cup type of Le- 

 clanche cell. It had been in constant use in 

 the laboratory for five months with no change 

 of electrolyte. As it polarized rapidly for 

 the first ten seconds after closing the circuit 

 through one or two ohms, its resistance was 

 measured with values of B of 40, 60 and 80 

 ohms. The values obtained were as follows : 



The average of these twelve determinations 

 is 1.389 ohms, and the mean variation from 

 this value is 0.002 ohm, while the probable 

 error of this result is 1 part in 2,600. 



But it is not my present purpose to discuss 

 experimental data except in so far as it shows 

 that Mance's method is not without some 

 merit. It has been shown that this method is 

 fully as accurate as is required for laboratory 

 use, whether the resistance to be measured be 

 of the first or second class. The purpose of 

 this paper will be fully attained if it has 

 clearly shown the principle underlying this 

 method, and pointed out the very obvious error 

 which has crept into many of the text-books 

 from Maxwell down to the present. 



Arthur W. S-mith. 



Physical Laboratory, 

 University of Michigan, 

 Ann Arbor, Mich., 

 February 11, 1905. 



ORGANISMS OX THE SURFACE OF GRAIN, WITH 

 SPECIAL REFERENCE TO BACILLUS COLI. 



The recent note by Dr. Erastus G. Smith 

 on the occurrence on grain of organisms re- 

 sembling the Bacillus coll communis^ appears 

 to warrant preliminary publication of some of 

 the results of my studies of the micro-organ- 

 isms normally present on the flowers and 

 fruit of certain plants in the Piedmont region 

 and the rice belt of South Carolina. These 

 studies, originally undertaken as a side issue 



' Science, May 5, 1905. 



