618 



ELECTRICAL MEASUREMENTS 



as a detector. To make an observation, the handle at the right 

 is turned until the galvanometer stands at zero. Then the lever 

 at the left is raised, causing the stile, through the medium of 

 the typewriter ribbon, to imprint a dot on the paper which is 

 carried by the large drum at the rear of the apparatus. When the 

 lever is lowered, a ratchet and pawl turn the drum a prede- 

 termined amount, and at the same time, by means of another 

 pawl and ratchet, actuated by an electromagnet, the contact 

 brush, B^, in Fig. 381, is advanced proportionally. 



FIG. 382. Potentiometer and registering apparatus for Rosa curve tracer. 



The teeth of the ratchets of the contact maker and of the drum 

 are numbered correspondingly, so that the brush and the record- 

 ing drum may be set at any desired position. As shown in Fig. 

 383, the points may be taken close together, so that irregular 

 waves may be dealt with. 



For the best work, it is necessary to connect the contact wheel 

 directly to the dynamo shaft and to keep the circuit conditions 

 perfectly constant. If this can be done, the Rosa curve tracer 

 furnishes the most accurate apparatus yet devised for mapping 

 periodic electrical phenomena. There are other modifications 

 of the contact method which reduce the time necessary for record- 

 ing the waves and give results sufficiently accurate for much 



