258 Electricity and Galvanism. 



causes it to fly up and again bring the platinum point and plate 

 in contact, when the same series of alternate attractions and re- 

 pulsions occur. In this way you see the brass bar rapidly vibrate, 

 and produce a loud humming musical sound, varying in pitch ac- 

 cording to the amount of amplitude of the vibrations, and cotem- 

 poraneously, a rapidly succeeding series of induced currents traverse 

 the coil of fine wire. If I now grasp in my hands a pair of brass 

 cylinders connected with the ends of the fine coil, a series of 

 currents of high intensity, and rapidly succeeding each other, rush 

 through the arms, producing a most painful and nearly intolerable 

 sensation. You observe that a bundle of iron wires is placed in the 

 hollow axes of the bobbin. The use of this is obvious enough, for 

 these wires becoming a series of powerful temporary magnets add 

 their inducing power to that of the initial current, and greatly in- 

 crease the tension of the excited electricity. Indeed, by withdraw- 

 ing the bundle of iron wire, you may diminish most materially the 

 severity of the shocks produced by this instrument, and thus enable 

 you very conveniently to adjust their force according to the case 

 under treatment. 



If you reflect for a moment on the principles on which the con- 

 struction of this very convenient arrangement is founded, you will 

 at once see that you cannot obtain by its aid a series of positive 

 and negative currents in a definite direction ; that neither of the con- 

 necting wires are capable of being regarded as negative and positive. 

 This you can readily understand from the results of the experiment 

 showed you just now with the galvanometer. Each of the conduct- 

 ing wires of this instrument convey alternately currents of opposite 

 characters. The wires, at the rate the bar is now vibrating, convey 

 about 500 currents per minute, each being alternately negative and 

 positive. To demonstrate the truth of this statement I have here on 

 a glass plate a piece of paper moistened with a mixed solution of 

 starch and iodide of potassium. I place on it the platinum extremi- 

 ties of the conducting wires of the electric-magnetic apparatus ; the 

 currents pass, electrolytic action occurs, the iodine is severed from 

 the potassium, and being set free, stains the starched paper. On 

 examining the paper you will find the purple stain of iodide of 

 amidine at both points where the platinum wires touched the surface. 



