4 INDUCTION SHOCKS 
of wire does not ordinarily exceed 600. The coil is 
mounted horizontally by one end upon a suitable sup- 
port. The ends of the wire are brought to two 
binding posts, situated at some convenient place on the 
support. 
The other coil, the secondary, consists of numerous 
turns of very fine insulated wire, wound upon a hollow 
spool whose inside diameter is such that the secondary 
coil can be brought over the primary. The number of 
turns of wire is usually between 5000 and 10,000. The 
length of the secondary coil is about equal to that of 
the primary. The ends of the wire are brought to 
binding posts mounted upon the spool. A slide, 30 or 
40 cm. long, projects from the support of the primary. 
The secondary is mounted upon this slide with its axis 
coincident with the axis of the primary. A scale, grad- 
uated in millimeters, is mounted on the slide. A pointer 
on the secondary coil is so placed that it indicates zero 
on the scale when the secondary covers the primary 
completely. A device for making and breaking the 
primary circuit automatically is usually included as 
part of the apparatus; and a bundle of soft iron wire, so 
constructed as to slide into the hollow core of the pri- 
mary coil, is likewise provided. 
Principle of the Inductorium. Whenever a steady 
current is flowing through the primary coil there exists 
about it a magnetic "field of force." This field may be 
