Experiments with Alternating Currents, 241 



vibrating wire were employed gave results of little utility. 

 An objection to the method is that the length of contact and 

 the position of contact (so to speak) with regard to the phase 

 of the alternating current are unknown. 



A well-designed alternator would provide a method of ob- 

 taining an alternating current almost perfectly harmonic in 

 character, and by means of an intermittent contact arranged 

 by the agency of the axis of the rotating armature the effect 

 of the bismuth on the alternating current could be studied 

 very efficiently. 



I employed a small alternator made from a motor, the 

 character of whose alternating current I do not know. 



To make the tabulated results given later more clear, it 

 may be mentioned that the wire of the bridge is divided into 

 1000 parts, and that increase in the reading means increase 

 in the resistance measured. 



The alternator gave a current with a frequency of about 

 30 or 40 per second. 



With constant current and permanent contact, the reading 

 was 489. 



The reading for minimum noise with the telephone was 

 about 501. 



The readings obtained with an intermittent contact and a 

 galvanometer are indicated in the table given below. I was 

 assisted by a friend, and they were taken rapidly. 



What is called the lead is the angle between the point of 

 intermittent contact and the point of zero current : in other 

 words, it is the angular distance the coil of the alternator has 

 advanced beyond the position in which no current is produced 

 before the galvanometer branch is made complete. 



Reading of Bridge which gave 

 Lead. no Deflexion of Needle of 



Galvanometer. 



-—5 roughly 405 approx. 



„ 485 „ 



10 „ 487 „ 



30 „ 489 „ 



90 „ 489 „ 



135 „ ... . 492 „ 



175 „ 475 „ 



180 „ . . . . . 482 „ 



225 „ 489 „ 



270 „ 489-3 „ 



315 „ 493 „ 



362 „ 477 „ 



