If, therefore, I rapidly make and break the current at b I produce an alternating current in the secondary coil. I will connect c and d with a miniature lamp and, resting a coarse file upon the free binding post, I will rake the end of the wire b up and down upon this file so that, as it dances along upon the file, it will rapidly make and break the primary circuit, and therefore rapidly change the strength of the magnetic field. You notice that the lamp lights up moderately well. It is being lighted by an alternating current. I move the wire a little more slowly and you see the flicker of the alternations. According to the label upon the lamp it requires ten volts, and our battery could not give that. We have therefore "stepped up" the voltage as we say and we have a veritable step-up transformer.
In this case the primary and secondary circuits are entirely separate. It is a familiar fact that different electric currents may pass through the same wire at the same time without apparent conflict. We send numerous telegraph despatches through the same wire at the same time. It is quite as easy for several pairs of persons to telephone over the same wire at the same time as it is for those same several pairs to carry on separate conversations in the same room at the same time, at, say, an "afternoon tea." We may use the same wire at the same time to carry direct and alternating currents. This fact was first discovered in 1902 by Bedell of Cornell University.
Primary and secondary currents do not require separate primary and secondary coils to convey them. They may or may not be connected into one continuous coil. It is quite immaterial whether they are connected or not so long as they are in the same magnetic field. Indeed, it seems that the field outside of the wire may be quite as important as the wire itself.
Fig. 119
We have now 100 turns in the primary and 200 turns in the secondary coils. Let us connect b with c so as to make one continuous circuit of 300 turns. Let us then put a branch upon b to connect with the battery, thus having 100 turns for the primary circuit, and put a branch upon a to connect with the lamp, thus having 300 turns upon the lamp, ([Fig. 119]). When now we rub b upon the file, as before, the lamp lights up more brightly than before, indicating that we have stepped up the voltage still higher. Varying the strength of the magnetic field induces a secondary current and the voltage of the induced current is determined, in part, by the number of turns in the secondary circuit. If what we have been saying is true we ought to be able to get these same results from an electric bell. To test this we connected wires with a and c, ([Fig. 120]), and since I knew that the secondary current at S would be too severe for the tongue we decided to feel it with the hands. For this purpose we want a larger surface than the wires themselves offer for contact with the hands, and so I twisted the bare end of each wire around an iron spike. The four boys then arranged themselves in line, joining hands, and the boy at each end of the line held a spike in his free hand. Thus we had put the enormous resistance of four human bodies joined in series in the secondary circuit. When now I connected two dry cells with a and b (P, [Fig. 120]) the hammer of the bell acted, like the file in the former case, as interrupter of the primary circuit. As it rapidly made and broke the primary circuit, it produced rapid changes in the strength of the magnetic field and thus induced a secondary current which the boys all felt. The fact that it forced its way through four bodies shows that its voltage was high. The high voltage was also indicated by the spark which always occurred in the bell. The primary circuit in this case has not more than three volts while the secondary has more than a hundred. We have it in our power to give the secondary current almost any voltage we choose, with this limitation each increase in voltage necessitates a proportional sacrifice of quantity. The watt power induced in the secondary circuit cannot exceed that contributed to the primary circuit—indeed cannot quite equal it since there is some loss in heat.
Fig. 120