THE SMITHSONIAN INSTITUTION. 335 



In the last three experiments the sulphuric acid was diluted with 

 five times its volume of water, and the specific gravity of the nitric 

 acid was 1.36. 



In the experiments with the Stohrer arrangement of Bunsen's 

 battery, acid like that of the first experiment with Deleuil's was 

 used ; the considerable difierence in resistance of elements in the two 

 experiments does not depend here upon the nature of the acid, but is oc- 

 casioned by the porous cells. In the second experiment with tlie Stohrer 

 battery, its own excellent cells were not used, but very brittle earthen 

 cells. By using these red earthen cells the resistance of the elements 

 increased three fold, from which we see what an important influence 

 clay cells have- upon the resistance of the element to induction, and 

 thus upon the force of the current. 



In Daniells' battery the red clay cells were also used ; in the first 

 experiment the zinc was placed in a mixture of 1 part sulphuric acid ^ 

 to 10 parts water ; in the Last experiment, acid which had been already 

 used, and still more diluted, was applied. 



To give the tangent compass a secure position, it was placed upon a 

 thick oak board built into the niche of a window, so that walking in 

 the room produced no vibration in the needle. Thick copper con- 

 ducting wires passed from the tangent compass to the wall, where they 

 were fixed over a door to a table on which the battery stood. 



The resistance of all this wire, together with the tangent compass, 

 is equal to 1.75; that is, it is equal to the resistance of a copper wire 

 1 millimetre thick, and 1.75 metre long. This resistance is in the 

 values of R in the above table, added to the essential resistance of the 

 elements ; thus the true values of R are always 1.75 less ; hence we 

 have. 



For the Deleuil battery — 



1. R nr 15.05 (10 water, 1 sulphuric acid.) 



2. R = 24.88 (acid used already and diluted more.) 



3. R = 5.85 (5 water, 1 sulphuric acid.) 



For the Stohrer battery — 



1. R =: 4.45 (white cells, ) 10 water, 



2. R = 16.25 (red cells, ) + 1 sulphuric acid. 



For the Daniells' battery — 



1. R = 9.35 (10 water, 1 sulphuric acid.) 



2, R = 19.75 (used acid, further diluted.) 



The resistance of the element depends upon the nature of the 

 liquid a;id the size of the pair of plates ; hence to be able to compare 

 the conducting capacity of different galvanic combinations properly, 

 the resistance must be reduced to the same sized pair of plates, and 

 thus the surface of the latter with which the experiment is made 

 must be known. 



The above mentioned galvanic elements have the following dimen- 

 sions : 



