RESISTANCE DEVICES 137 



Low -resistance Shunts for Use in Alternating-current Meas- 

 urements. In many of the modern methods for alternating- 

 current measurements, for instance, those for determining the 

 ratios and phase angles of current transformers and for measuring 

 power by the electrostatic wattmeter, " non-inductive " shunts 

 having large carrying capacities are employed. 



It is necessary that the inductance be reduced to a minimum, 

 for the phase displacement between the current and the P.D. 

 at the shunt terminals must be as small as possible and the im- 

 pedance must be sensibly the same as the direct-current re- 

 sistance. Also, when the inductance is reduced to a minimum 

 the stray field and therefore the disturbing effect on neighboring 

 instruments is correspondingly decreased; this may be of import- 

 ance when large currents are dealt with. 



If low resistances are employed, the amount of inductance 

 which can be tolerated is exceedingly small. For instance, at 60 

 cycles per second a phase displacement of 0.04 will be produced 

 in a resistance of 0.001 ohm by an inductance of 0.000000002 

 henry or 0.002 microhenry or 2 cm. c.g.s. At low power factors 

 even this inductance is of importance when power measure- 

 ments are made with the electrostatic wattmeter. These small 

 inductances, having a magnitude of only a few centimeters, can 

 be attained only by special care in design. 



As such low resistances are for use with large currents and the 

 voltage drop in them is likely to be considerable, special means 

 must be provided for dissipating the heat generated. If air 

 cooling is relied upon the shunt becomes bulky and expensive, 

 so it is generally immersed in oil which is violently stirred and 

 is kept cool by a water jacket, as in the Reichsanstalt form, 5 

 or the shunt is made in the form of a tube through which water is 

 briskly circulated, as in that designed at the National Physical 

 Laboratory, London. 4 



Two sizes of the Reichsanstalt shunts are shown in Fig. 62. 

 They are made according to a suggestion originally due to 

 Ayrton. A single thin strip of manganin is doubled back on 

 itself at the middle of its length, and the two parts separated by 

 a very thin layer of mica insulation. By this means the area and 

 consequently the flux included l>y the circuit are reduced to a 

 minimum. To obtain a small inductance and absence of skin 



