RESISTANCE DEVICES 



141 



they are terminated by binding posts. This cuts down the stray 

 field of the shunt itself and brings the current leads so close to- 

 gether, that their field has little effect on the neighboring appa- 

 ratus. A thin copper ring is hard-soldered to the tube at the mid- 

 dle of its length. When the shunt is used in connection with the 

 electrostatic wattmeter, this ring is used as a terminal (see Fig. 

 188). 



The tube is covered with a layer of varnished cambric about 

 0.2 mm. thick and outside this are the potential leads, in the form 

 of thin sheaths of copper foil about 0.04 mm. thick extending 

 from the ends of the tube to near its middle, where they are 

 terminated in potential posts. 



The fluxes which produce inductive effects are those in the 

 insulating medium between the tube and the potential leads and 

 in the main tube itself. The insualtion therefore should be as 

 thin as practicable and the potential leads should very closely 

 surround it. The resistance tubes should be very thin and of 

 large diameter. As the resistivity is high, the skin effect is 

 negligible, less than 1 part in 10,000 at ordinary frequencies. 

 This construction reduces the effective inductance to 3 or 4 cm. 

 or to 0.003 or 0.004 microhenry. 



DATA CONCERNING WATER-COOLED MANGANIN RESISTANCES DESIGNED 

 AT THE NATIONAL PHYSICAL LABORATORY 



To obtain a high carrying capacity, water from the city mains is 

 briskly circulated through the resistance tube at a rate of about 

 15 liters per minute. The formation of a layer of hot water in 

 contact with the resistance material is prevented by a centrally 

 located glass rod which nearly fills the tube. For the same 

 change in resistance due to heating, approximately three times 

 as much energy may be dissipated as when simple air cooling is 



