142 



ELECTRICAL MEASUREMENTS 



relied upon. As much as 10 kw. may be dissipated by a current 

 of 3,000 amp. in a tube 1^ in. in diameter and 18 in. long. The 

 current density may be as great as 16,000 amp. per square inch. 

 For manganin tubes, up to 1.5 mm. thick, 10 watts per square 

 centimeter may be allowed as the working load. 



To minimize the possibility of accidents, this form of resistance 

 is used in a vertical position, the water inlet being at the bottom 

 so that the tube is always filled. 



Resistance Boxes. Generally speaking, resistance boxes are 

 constructed so that the resistance between their terminals may 

 be varied from zero up to the full capacity of the box by 0. 1 ohm 

 or by 1-ohm steps. This must be accomplished by the use of a 



A r, A n_Z 



I PS Si 



?-%,. * * 



FIG. 66. Diagram show- 

 ing series arrangement of re- 

 sistance coils. 



FIG. 67. Connection block 

 for resistance box. 



moderate number of coils. A common arrangement is to employ 

 coils of 1, 2, 2, 5 ohms, with similar sets for the tenths and the 

 tens, hundreds and thousands. Another possible arrangement 

 is based on coils having the denominations 1, 2, 3, 4. The coils 

 are so mounted that any desired resistance is obtained by placing 

 coils in series, as shown in Fig. 66. The drawing of a plug re- 

 moves the short-circuit on the corresponding coil. The current 

 must then pass from the terminal block through the coil, and on 

 to the next block. 



It is essential that the top of a resistance box be very rigid in 

 construction. Consequently, the blocks must be firmly screwed 

 to the vulcanite top. The taper of the plug should be such 

 that a good contact may be obtained without the plug becoming 

 wedged in place. The blocks should be undercut, as shown in 

 Fig. 67, so that each may be thoroughly' insulated from its 



