30 ELEMENTARY PHYSIOLOGY. IV. 



the contractions should decrease in the same ratio as 

 they previously increased. 



In this experiment there is no difficulty in obtaining 

 the main result, viz. that the difference in the strength of 

 the current required to produce a minimal and a maximal 

 contraction is small, but it will probably be found that 

 the minimal current (threshold) varies slightly, and that 

 the same height of contraction is not constantly ob- 

 tained with the same position of the secondary coil. 

 This is due partly to an increase of irritability caused 

 by the stimulus, and partly to the difficulty of opening 

 and closing the key at the same rate, i.e. to the difficulty 

 of keeping the induced current of constant intensity. 



Repeat the stimuli, pushing up the secondary coil a 

 centimetre at a time. A large increase in the strength of 

 the stimulus will cause no increase in the height of the 

 contraction. When the induction current is very strong, 

 there will be an increase in contraction height, due to 

 the break induction current stimulating at the break as 

 well as at the make. 



2. Fusion of single contractions to form 

 tetanus. Arrange for single induction shocks, intro- 

 ducing into the primary circuit, in place of a key, an 

 oscillating rod and mercury cup. This is a thin band of 

 steel about 35 cm. long and 1 to 2 cm. wide; at one end, 

 at right angles to the band (Pig. 13), is fixed a pointer, 

 about 3 cm. long. The band can be fixed at any part of 

 its length by a wide clamp and connected with the 

 clamp is a binding screw. A cup containing mercury is 

 placed underneath the pointer and the clamp is arranged 

 on the stand at such a height that when the band de- 

 scends in oscillation the pointer dips into the mercury. 



