oC 
[MILLER & KENRICK] ELECTRICAL MEASURING APPARATUS 99 
If the relay contact r be closed, a current flows through the solenoid 
magnet À (see fig. VI.) which forces the wheel 4 against Z and W, as 
shown in fig. V., thus com- 
municating the motion of Z 
to W and increasing the re- 
sistance between S and [8]. 
The relay contact / operates 
the solenoid Z and decreases 
the resistance. (See fig. IIT.) 
A friction block B stops the 
wheel promptly when the solenoid ceases to act. P is a pointer attached 
to the axis of W, which moves in front of the dial D, divided into hun- 
dredths. The flywhecl #' serves to gear down the motor and to carry 
the commutator H, 


THE STATIONARY RESISTANCE COILS. 
At the back of the dial are fastened a number of resistance coils, the 
ends of which communicate electrically with: mercury cups arranged in 
two rows in the wooden 
beam À (the ‘“switch- 
board”). Pieces of stout 
copper wire fastened on the 
“sign blocks” G (see also 
fig. VII.) dip into the mer- 
cury cups and make the 
necessary connections, so 
that, for example, the act 
of inserting thesign “Volts” 
above the dial converts the 
apparatus into a Voltmeter. 
The following diagram 
(table I) shows the arrange- 

To To To 
25) +4] (or) 

es 
Fic. VII. 
ment of the resistance coils. 
7o (OV) The figures in square brac- 
Fie. II. kets refer to the numbers of 
the mercury cups; those in round brackets refer to binding posts on the 
bed of the machine ; the remainder give the resistance in ohms of the 
various coils. 
