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. [Illustration: Fig. 120] Suppose we operate a bell on a primary current having three volts and .25 ampere, that is, .75 watt. Suppose then the voltage of the secondary current is stepped up to fifty times three, or 150 volts. The quantity of secondary current will be found to be somewhat less than one fiftieth of .25 or .005 ampere. The 150-volt alternating current from the bell is more tolerable than that from a 150-volt dynamo, because the quantity is limited in the former case.