6o8 THE POPULAR SCIENCE MONTHLY 



I = E/(r + E). (5) 



The experiments by which these results were established constitute 

 the most important of Ohm's work and they well repay careful study. 

 After some preliminary experimentation his apparatus was reduced to 

 the following essential parts: a Cu-Bi thermo-couple for generating a 

 steady current and a specially constructed magnetic torsion balance for 

 measuring the current strength. The apparatus and scheme of connec- 

 tions is shown in Fig. 3. 



During the earlier part of his experimental work one thermo- 

 juncture was surrounded by a steam-jacket and the other by an ice- 

 jacket; during the latter part of the work the hot juncture was left at 

 room temperature. The procedure for the experiment was as follows: 

 The steam and ice jackets were first brought to their respective tem- 

 peratures, the length of the test wire was then adjusted to the required 

 length and lastly the torsion head turned until the magnetic needle was 

 brought back to its zero position. The reading of the torsion head was 

 then recorded and the experiment repeated with a new length of wire. 



With the completion of these experiments Dr. Ohm had established 

 both parts of his law, and may be said to have solved the problem to 

 which he had set himself. As in the case of Sir Isaac Newton and the 

 law of gravity, Ohm now found himself in the possession of a key to 

 many doors closed to previous workers, and he proceeded at once to use 

 it, as is shown by his theoretical paper of 1836 and his book of 1827. 



V. This paper was published in the Poggendorf Annalen, is ten 

 pages long and is entitled "Versuch einen Theorie der durch galva- 

 nische Krafte hervorgebrachten elektroskopischen Erscheinungen." It 

 is a purely theoretical paper and foreshadows the book which he wrote 

 the following year. In this paper Ohm enunciates his complete law, 

 contrary to the widely accepted statement that the law was first given 

 in the book of 1827 {e. g., Eeed and Guthe, "College Physics," 1911). 

 It also contained the correct formula based on this law, for the change of 

 terminal potential difference due to a change in the external resistance. 

 Finally the application of the law to many practical problems is 

 discussed. 



VI. The year 1827 furnishes the final paper of the series upon the 

 galvanic circuit, followed by the appearance of the book elaborating the 

 newly discovered relations. This paper appeared in Schweigger's 

 Journal and was entitled " Einige elektrische Versuche." It is a paper 

 of eight pages and contains the results of two experimental investiga- 

 tions confirmatory of the work of the previous year, as follows: (1) A 

 verification of the conclusion as to the uniformity of the distribution of 

 current over the cross-section of the conductor; (2) a verification of the 

 formulas for the combination of resistances in parallel. This may be 



