JO 



DIRECT CURRENT GENERATORS. 



[Exp. 



APPENDIX I. 

 MISCELLANEOUS NOTES. 



14. Current and Voltage Adjustment. For currents of small 

 values, when a wide range of adjustment is desired, a series resist- 

 ance (Fig. i) is frequently inadequate and it is better to shunt off 

 current from a resistance R, as in Fig. 4. 



FIG. 4. FIG. 5. 



Methods for adjusting voltage or current. 



By adjusting the slider p, the voltage delivered to the apparatus 

 under test can be given any desired value from zero up to the value 

 of the supply voltage. A modification which is sometimes conveni- 

 ent employs two resistances, B and C, Fig. 5. The adjustment is 

 made by short circuiting or cutting out more or less of one resistance 

 or the other, but not of both. The full amount of one resistance 

 should always be in circuit. 



15. Temperature Corrections. The conductivity of copper varies 

 with temperature, according to the law given below. Resistance 

 values to be significant should therefore be for some specified tem- 

 perature; known for one temperature they can be computed for any 

 other. Temperature rise can be computed from increase in resist- 

 ance. In all cases where accuracy of numerical results is important, 

 as in commercial tests for efficiency, regulation, etc., definite tempera- 

 ture conditions should be obtained ; for this the detailed recommenda- 

 tions of the A. I. E. E. Standardization Rules should be consulted. 

 To meet standard requirements, a run of several hours is commonly 

 required. In practice work this is not necessary, it being usually 

 sufficient to specify resistances as cold when taken at the beginning 

 and hot when taken at the close of the test. 



Let R t be the resistance of a copper conductor at a temperature 



