14 INDUCTION SHOCKS 
a source of great modification of stimuli. Finally, as we 
have seen, there is a difference in physiological effect 
between make shocks and break shocks. 
Of the sources of variation just described the following 
are subject to laws which are determinable, and are to 
be included, therefore, in our quantitative scheme: The 
construction of the inductorium, the position of the sec- 
ondary coil with respect to the primary, the presence or 
absence of an iron core in the primary, the intensity and 
voltage of the primary current, the use of make or break 
shocks, the electrical resistance of the stimulated tissue 
and the mode of contact of the stimulating electrodes 
with the tissue. 
The variable which is not determinable is the effect 
on the stimulus of the manner of making or breaking 
the primary circuit. This must be, so far as possible, 
made uniform. 
Methods Previously Proposed. The first attempt to 
measure induction shocks is said to have been made by 
Rosenthal in 1 85 7 .* Two years later Pfliiger made quan- 
titative comparisons between shocks, varying their in- 
tensities by varying the primary current, leaving all 
other factors constant. His method gives accurate rela- 
tive results, but seems not to have commended itself to 
physiologists, probably because it calls for a rather com- 
* See Garten: Handbuch der physiol. Methodik, 1908, Bd. II, 
Abt. 3, S. 393. 
