606 THE POPULAR SCIENCE MONTHLY 



In the light of Ohm's later work it is easy to see that this formula 

 is absurd, a conclusion indeed soon reached by Ohm himself. It how- 

 ever marks one of the steps in the discovery of the law, and an examina- 

 tion of the experimental data shows that it represented these data very 

 closely. Ohm was not however to be long misguided by an equation 

 which could be easily checked by increasing the range of the experi- 

 ment. He saw that, no matter how precise equation (3) might prove 

 as an approximate formula, it could hardly represent a law of nature. 

 He therefore prepared to test an external resistance fifteen hundred feet 

 long. He does not seem to have published the result of this experiment, 

 but he must have seen his mistake, for he promises to develop a new 

 and correct equation. This he did a year- later. 



II. Two more papers were published by Ohm in the year 1825, as 

 follows: "IJber Leitungs-fahigkeit der Metalle fur Elektricitat," con- 

 taining a preliminary announcement of his studies on the relative con- 

 ductivities of different metals. The results were published the follow- 

 ing year and are considered under paper IV. 



III. "Ueber Electricitatsleiter." This was a discussion of the dis- 

 crepancy between the results of Barlow and Becquerel, together with 

 the acknowledgment of the inaccuracy of his own formula for the " loss 

 of force " and an intimation of his intention to revise the formula. 

 This closes the work for the year 1825. During these experiments he 

 used for the source of current a Cu-Zn cell and measured the potential 

 difference by means of a Coulomb torsion balance. The progress of 

 the work is marked by a clearing of the way, by a grasp of the problem 

 and a development- of method rather than by positive achievement. 



IV. Two papers appeared during the year 1826 : the first of these 

 was by far the most important and was a long one of twenty-nine pages, 

 entitled " Bestimmung des Gesetzes, nach welchem Metalle die Contact- 

 Electricitat leiten, nebst einem Entwurfe zu einer Theorie des Volta- 

 ischen Apparates und des Schweigger'schen Multiplicators." While 

 not so well known as his book on the mathematical theory of the circuit, 

 published a year later, it is in reality his most important work and con- 

 tains the following results. 



1. The data on relative conductivities promised in paper II. Ohm 

 measured the relative conductivities of copper, gold, silver, zinc, brass, 

 iron, platinum, tin and lead. Without going into the details of these 

 experiments it is important to note that Ohm had gained a thorough 

 appreciation of the significance of the specific conductivity of a 

 conductor. 



2. Experiments showing that the resistance of two conductors is the 

 same when they have the same ratio of length to cross-section. Ohm's 

 first experiments on cross-section proved the proposition which had 

 already been proven by Davy and Becquerel, namely, that the resistance 



