246 THE MAGNETIC CIRCUIT [ART. 70 



distance h between the coils. Hence, a constant weight Q 

 regulates for a constant current. When the coils are further 

 from each other, the induced secondary voltage is less, on account 

 of a much higher leakage flux. When the current increases 

 momentarily, due to a decreasing line resistance, the coil is 

 overbalanced and rises till the induced voltage and current fall 

 to the proper value. Thus, the coil always floats at the proper 

 height to induce the voltage needed on the line. 



FormulaG (173) and (174) apply also to the mechanical forces 

 between the primary and the secondary coils of a constant- 

 potential transformer (Figs. 13 and 51). Under normal con- 

 ditions these forces are negligible, but in a violent short-circuit 

 the end-coils are sometimes bent away and damaged, unless 

 they are properly secured to the rest of the winding. Such 

 short-circuits are particularly detrimental in large transformers, 

 having a close regulation, that is, having a very small internal 

 impedance drop, and which are connected to systems of practically 

 unlimited power and constant potential. As Dr. Steinmetz puts 

 it, the closest approach to the appearance of such a transformer 

 after a short-circuit is the way two express trains must look 

 after a head-on collision at high speed. 



Another interesting example of the effect of the mechanical 

 forces produced by a magnetic field is the so-called pinch phe- 

 nomenon. 1 The lines of force which surround a cylindrical con- 

 ductor may be compared to rubber bands, which tend to com- 

 press it. With a liquid conductor and large currents, such 

 for instance as are carried by a molten metal in some electro- 

 metallurgical processes, the pressure of the magnetic field is 

 sufficient to modify and to reduce the cross-section of the liquid 

 conductor. This was first observed by Mr. Carl Hering and 

 called by him the pinch phenomenon. In passing a relatively 

 large alternating current through a non-electrolytic liquid con- 

 ductor contained in a trough, he found that the liquid contracted 

 in cross-section and flowed up-hill lengthwise in the trough, 

 climbing up on the electrodes. With a further increase of 



1 E. F. Northrup, Some Newly Observed Manifestations of Forces in the 

 Interior of an Electric Conductor, Physical Review, Vol. 24 (1907), p. 

 474. This article contains some cleverly devised experiments illustrating 

 the pinch phenomenon, and also a mathematical theory of the forces which 

 come into play. 



