Technikatörténeti szemle 10. (1978)

A MÉRÉS ÉS A MÉRTÉKEK AZ EMBER MŰVELŐDÉSÉBEN című konferencián Budapesten, 1976. április 27–30-án elhangzott előadások II. - Kind, D.: Az elektromos mértékegységek fejlődése a SI rendszerben

for electric resistance 1 ohm = 10

9 e.m. CGS units (1 e.m. CGS unit of resistance = 1 cm s

_i

), represented by the direct current resistance of a mercury column of constant cross-section at the temperature of melting ice, having the mass 14,4521 g and the length 106,300 cm; . for electric current 1 ampere = 10

_1 e.m. CGS units (1 e.m. CGS unit of current = 1 g 1

/

2 cm

1

/

2 s

_1

), represented by a direct current depositing at a rate of 0,00118 g per second silver from a solution of silver nitrate in water; for voltage 1 volt = 10

s e.m. CGS units (1 e.m. CGS unit of voltage as 1 g 1

/

2 cm 3

/

2 s

_1

), represented by the part of the voltage of a Clark cell. With these decisions there in fact existed each two definitions for the electrical units. Consequently, from thereon one was distinguishing between the ,.absolute" units—as units linked with the CGS system—and the „international" units—as units determined by rules for their realization. These resolutions seem to us, from our present view, to be unsuitable, because, due to Ohm's law, only two units can be chosen freely. In fact, from this overdetermination soon resulted considerable discrepancies. National legislations till to the adoption of the international electrical units in 1908 To comply with the urgent need, the United States and Great Britain promulgated the relevant laws on electrical units already one year after the Congress of Chicago. Thus, like France in a decree of 1896, they adopted the recommendations on the three international units. The over-determination of these resolutions led however to difficulties, as from ameliorated measurements resulted that the adopted value for the voltage of the Clark cell had been too high by 0,1%. Germany did avoid this over-determination in its law Gesetz betreffend die elektrischen Maßeinheiten, enacted in 1898, and did limit the determination of the realization of legal electrical units of ohm and ampere; the volt had been defined as that electromotive force producing in a conductor with the resistance of 1 Q an electric current of 1 A. To retain the linkage to the CGS system, it was endeavoured to compare the legal ohm to the „absolute" ohm as precisely as possible. For that reason, the length of the mercury column representing the ohm, whose mass had been determined in Chicago in 1893, had to be measured as precisely as possible. To this figure 5 shows as historical document the letter from Professor G. Wiedemann, Leipzig, of 14 January 1892, addressed „An den Präsidenten der PhysikalischTechnischen Reichsanstalt, Herrn wirklichen Geheimen Rath Professor Dr. von Helmholtz, Excellenz". In this letter he is confirming the length of 106,3 cm

—later on