Technikatörténeti szemle 9. (1977)

A MÉRÉS ÉS A MÉRTÉK AZ EMBERI MŰVELŐDÉSBEN című konferencián Budapesten 1976. április 27–30-án elhangzott előadások I. rész - Debreczeny Á.–Vajda P.: A Bláthy-féle fogyasztásmérő, az áramfogyasztás gyakorlati mérőmódszerei

moment is set up in the disc, which is proportional to the product of currents exciting the coils — i. e. approximately to the power taken up by the load. This device was already superior to Shallenberger's ampere hour meter but it could not be regarded to be a watt-hour meter proper: it failed to provide for an accurate 90° phase shift between the magnetic fields generated by the voltage and the current. At any rate, it approximated the requirement of the torque moment acting upon the disc being proportional to the electric power passing through the device (taken up by the load). Blathy wished to improve his experiments' started along correct principles. By an ingenious spatial arrangement of the magnets, he managed to pass only the small friction of the flux of magnet excited by the mains voltage through the rotary disc; the major friction was claused through a magnetic by-circuit. In this way, the phase position of the flux intersecting the disc could be adjusted so that it was shifted in phase by exactly 90° (or nearly so) relative to the magnetic field of the current. To brake the rotary disc, Blathy utilized the eddy-current brake of the permanent magnet even in the first watt-hour meter. Its braking moment is linearly proportional to the angular velocity of the disc, so in a dynamic equilibrium, the angular velocity of the disc is proportional to the power passing through the meter. The first workable model of Blathy's watt-hour meter was manufactured for about 20 years. Then, in about 1912, another event of world-wide significance occurred: the Ganz Factory introduced a new watt-hour meter marked „Bi". In contrast to the heavy weight of the first Blathy watt-hour meters (20 kg or more), the Model „Bi" meter was as light as 2,6 kg. A universal shortcoming of watt-hour meters at that time was their accuracy of measurement varying with the mains voltage. In other words, the watthour meter performed accurate measurements at a given mains voltage only. Blathy eliminated this additional error through increasing the magnetic inductance in the voltage magnet beyond the „bend" of the magnetization curve. As a result, the magnetic flux of the voltage circuit varies nearly linearly with the excitation voltage. Thus the error of measurement became unaffected by variations in the mains voltage. Similarly, the error of measurement could be made independent, to a large extent, of the mains frequency. The new watt-hour meter had two features that may be rightly called ,,epoch-making" — it could be disassembled and overloaded. Blathy devised the new watt-hour meter so that, in a re-assemblage, each component part returned necessarily and exactly to its original position, thus eliminating the need for a subsequent alignment. This facility was not offered by any other type of watt-hour meter. The other valuable feature was the overload capacity. Magnetic fields, unidirectional as well as alternating ones, exert a braking effect upon the rotary disc. Blathy eliminated this error by a special arrangement (patented in 1912) in which a by-circuit was attached to the poles of the main-circuit magnet, the magnetic impedance of which was increasing with the loading. Thus the bycircuit detracted a great part of the flux from the effective air gap at low loads (associated with a low magnetic field strength). On the other hand, with increas-