APPLICATIONS OF THE GRAPHIC METHOD. 177 
A good knowledge of the subject of electricity is especially 
valuable to the student of physiology. But there are a few ele- 
mentary facts it is absolutely necessary to bear in mind: 1. An 
induced current exists only at the moment of making or break- 
ing a primary (battery) current. 2. At the moment of making, 
the induced current is in the opposite direction to that of the 
primary current, and the reverse at breaking. 3. The strength 
of the induced current varies with the strength of the primary 
current. 4. The more removed the secondary coil from the 
primary the weaker the current (induced) becomes. 
The clock-work mechanism and its associated parts, as seen 
in Fig. 174, on the right, is usually termed a myograph. 
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a 
Fia. 174.—Arrangement of apparatus for transmission of muscular movement by tambours 
(after McKendrick). a, galvanic element; b, primary coil; c, secondary coil of inducto- 
rium ; d, metronome for interrupting primary circuit when induction current is sent to 
electrodes k ; h, forceps for femur ; the muscle, which is not here repr d, is attached 
to the receiving tambour g, by which movement is transmitted to recording tambour e, 
which writes on cylinder jf. 
Instead of muscular or other movements being communi- 
cated directly to levers, the contact may be through columns 
Fie. 175.—Tambour of Marey (after McKendrick). a, metallic case ; b, thin India-rubber mem- 
brane; c, thin disk of aluminium supporting lever d, a small portion of which only is repre- 
sented ; e, screw for placing support of lever vertically over c ; f, metallic tube communi- 
cating with cavity of tambour for attachment to an India-rubber tube. 
12 
