TRANSFORMERS. [Exp. 



the average value. It is seen that the maximum value of the flux 

 depends upon the average and not the effective value of electromotive 

 force. If we let the form factor / designate the ratio of effective to 

 average value, E eff- = / av< and 



Bmax - = \fnSA~ ' 

 For a sine* wave /=i.i, substituting which gives the formula of 34. 



APPENDIX III. 



USE OF A WATTMETER, VOLTMETER AND AMMETER; ARRANGE- 

 MENT OF INSTRUMENTS AND CORRECTIONS TO BE APPLIED. 



36. The ordinary measuring instruments consume, in themselves, 

 a small amount of power, usually only a few watts. In many cases 

 this can be neglected, particularly in testing apparatus requiring 

 considerable power, but in precise measurements of small quantities 

 the effect of these losses should be considered. So far as the load 

 run in the present experiment is concerned, the losses in instruments 

 can be neglected; the errors and the methods of correcting for them 

 should, however, be noted for use whenever necessary. 



Some arrangements of instruments introduce larger errors than 

 others. Furthermore, the errors can be readily corrected for in some 

 cases and not in others. 



In selecting a method for arranging instruments, choose one in 

 which the errors (even if large) can be best corrected for, or else 

 choose one in which the errors are as small as possible and no cor- 

 rection is necessary. So far as convenience is 'concerned, the latter 

 is to be preferred. 



37. The Wattmeter. A wattmeter has two coils : a series or 

 current coil, connected in series with one line of the circuit as an 

 ammeter, and a shunt or potential coil, connected in shunt, from one 

 line to the other, as a voltmeter. 



* (35*). For a sine wave, E av =* max ; and eff = -^ E^. See 



page 37, Bedell and Crehore's Alternating Currents. Form factor is 

 f = E eft -^ av = i.i for a sine wave. (Form factor was first used by 

 Roessler as av . H- eff ., which for a sine wave is .9.) 



