The first stage of ohm experiment is to explore the relationship between the electromagnetic force attenuation caused by current and the length of wire. The results were published in his first scientific paper 1825 in May. In this experiment, he encountered difficulties in measuring the current intensity. Inspired by the galvanometer invented by German scientist Schveger, he skillfully combined the discovery of magnetic effect of current with the method of library torsion balance and designed a current torsion balance. It is used to measure the current intensity. Ohm, from the preliminary experiment, the electromagnetic force of current is related to the length of conductor. Relationship is not directly related to the expression of ohm's law today. Ohm did not relate the potential difference (or electromotive force), current intensity and resistance at that time.
Before Ohm, although there was no concept of resistance, some people had studied the conductivity of metals. Ohm works hard. 1825 In July, Ohm also studied the relative conductivity of metals with the device used in the previous experiment. He made wires with the same diameter from various metals and measured the relative conductivity of metals such as gold, silver, zinc, brass and iron. Although there are still many mistakes, Ohm thinks that the fact that the current is constant in the whole wire shows that the current intensity can be used as an important basic quantity of the circuit, and he decides to study it as a main observation in the next experiment.
In the previous experiment, the battery used by Ohm was a voltaic pile, but the electromotive force of this pile was unstable, which made him very headache. Later, it was suggested to use bismuth-copper thermocouple as power supply, thus ensuring the stability of electromotive force of power supply.
1826, Ohm deduced his law by using the experimental device shown above. A current torsion balance is installed on the wooden base, DD' is the glass cover of the torsion balance, CC' is the dial, S is the magnifying glass for observation, M and M' are the mercury cups, abb'a' is the bismuth frame, and one leg of the bismuth frame and the copper frame contact each other to form a thermocouple.
Ohm prepares conductors with the same cross section but different lengths, connects each conductor into the circuit in turn for experiment, observes the deflection angle of the torsion drag pin, and then repeats the operation under different conditions. According to the experimental data, the following relations are summarized:
X=q/(b+l) where x represents the current flowing through the wire, which is proportional to the current intensity, A and B are two parameters of the circuit, and L represents the length of the experimental wire.
Ohm published a paper in April 1826, rewriting Ohm's law as follows: x=ksa/ls is the cross-sectional area of a conductor, k is the conductivity, a is the potential difference between two ends of the conductor, l is the length of the conductor, and x is the current intensity passing through L. If the resistance l'=l/ks is substituted into the above formula, X=a/I' is obtained, which is Ohm's law.