CHAPTER IX 
THE MAKE AND BREAK OF THE PRIMARY CIRCUIT 
FROM the beginning of the use of induction shocks for 
stimulating living tissues investigators have recognized 
that the physiological intensities of these shocks are 
markedly affected by the manner of making or breaking 
the primary circuit. Helmholtz * called attention to 
this fact in his study of induced currents, and in the dis- 
cussion of the variable factors to be considered in the 
attempt to measure induction shocks (p. 14), I pointed 
out that the manipulation of the primary key is a vari- 
able whose influence cannot be mathematically deter- 
mined, and which, therefore, must be made as uniform 
as possible. 
Before entering upon a discussion of means whereby 
the manipulation of the primary make and break key 
can be made uniform, it is desirable to point out briefly 
the manner in which variations in the break and make 
of the primary circuit modify stimulating intensities. 
In the account of the theoretical basis for the break 
M 
shock formula, Z = - / (p. 34), the statement was 
Lt 
* Helmholtz: Poggendorf's Annalen der Physik und Chemie, 1851, 
Ixxxiii, p. 538. 
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