ELECTRICITY METERS 521 



the disc constant of the watt-hour meter and the gear ratio of 

 the contact arrangement which is described later, the demand 

 may be calculated. 



Fig. 307 shows the general appearance of one of these devices. 

 A very powerful, double-spring clock or an electrically driven 

 clock is used to drive the drum D over which the paper tape T 

 is passed ; to prevent slipping of the tape the drum is armed with 

 needle points. By the clockwork the tape is drawn from the 

 magazine M and caused to pass in front of the punch P. W is 

 the take-up roll and is actuated by a friction drive from the clock. 

 M f is the magazine roll for the strip of carbon paper used in 

 impressing the record on the tape. 



Whenever a current is passed through one of the magnets of 

 the punch P the corresponding armature is drawn in and a 

 mark is made. Once each hour a reference mark is printed on 

 the tape. 



To actuate the punch a contact arrangement is added to the 

 counter of the watt-hour meter; it is shown diagrammatically in 

 Fig. 308. The wheel g is driven by . j i 



the counter and revolves once for each 

 100 revolutions of the meter disc. 

 The contactor C and the weight W 

 are in one piece, which is loose on the 

 shaft. The pin P is long enough to 

 engage with this piece and then to 

 push it to the dotted position, when ? Energy 

 it suddenly falls forward. This causes FIG. 308. Diagram of 



the contactor C to connect b and b' contacts for Ingalls relay 



. .... demand indicator, 



lor an instant, thus closing the circuit 



through the magnets of the punch, which then marks the tape. 

 After 100 revolutions of the meter disc this operation is repeated. 

 The appearance of the record thus obtained is seen in Fig. 309. 



The tapes are replaced once a week; when this is done the time 

 of the beginning and the ending of the record is recorded on the 

 tape. 



To find the maximum demand the tape is examined and those 

 parts where the marks appear to be closest together are selected 

 for measurement. A scale having a length corresponding to the 

 movement of the tape during % hour is applied, and the number 



