tongue about i cm. apart. Then close the circuit. Xote the effect. 

 Is there any perceptible difference between anode and cathode? Ex- 

 plain. 



1/7. The rheocord. A device for introducing resistance into a 

 circuit or for obtaining fractions of the electromotive force of a 

 cell. Lead wires from the dry cell through a key to posts o and I 

 pnd then from post o and the slider to platinum electrodes. Place 

 the electrodes in salt or acid solution and determine the relative 

 amount of electrolysis when the slider is moved toward the o or i 

 post. Draw a diagram showing the course of the current. 



178. Induction coil. Induced currents are usually employed for 

 stimulation. These are momentary currents which appear in any 

 circuit when a current in an adjoining circuit is made or broken, 

 or its intensity altered. In the instrument the wires of the two 

 circuits are arranged in two parallel coils primary circuit ( induc- 

 ing circuit) and secondary (induced) circuit to intensify the 

 effects. An automatic interrupter is inserted in the primary circuit. 

 Study the instrument and make a diagram showing its essential con- 

 struction. Place the primary coil in circuit with a simple key 

 and dry cell. 



a. Direction of induced currents. Attach wires to the secondary 

 coil and apply the ends to starch iodide paper. Attach primary 

 circuit wires for single shocks. Close and open the key in primary 

 circuit a number of times. Effect? Note the cross circuiting key 

 at the poles of the secondary coil. Its purpose? Apply the elec- 

 trodes to test paper as before and close the key in the primary circuit 

 several times in succession but cross circuiting the secondary coil 

 each time before opening. Result? Repeat, cross circuiting before 

 making so as to allow only the break induced current to pass through 

 the electrodes. Result? \Yhat conclusions do you draw regarding 

 the direction of the induced current on making and on breaking 

 respectively ? 



b. Separate the primary and secondary coils to some distance. 

 Close the primary circuit and then place the secondary electrodes 

 (platinum-tipped wires attached to secondary coil) on the tip of the 

 tongue. Any result? Now make and break several times. Result? 

 Slide secondary nearer primary, testing as before. Note the effect 

 on the intensity of the shock. \Yhich is stronger, make or break 

 shock? Explain the difference in intensity of shock. Place the coils 

 at right angles to each other. Are shocks perceptible? Change the 

 angle between the coils, and test strength of shocks. Give a generali- 



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