Ten minutes later, with but a single officer now accompanying them, Tho Stan Drel, the terrestrial scientist, and the Talsonian scientist were inspecting the power installation. They had entered a large stone building, into which led numerous very heavy silver wires. The insulators were silicate glass. Their height suggested a voltage of well over one hundred thousand, and such heavy cables suggested a very heavy amperage, so that a tremendous load was expected. Within the building were a series of gigantic glass tubes, their walls fully three inches thick, and even so, braced with heavy platinum rods. Inside the tubes were tremendous elements such as the tiny tubes of their machine carried. Great cables led into them, and now their heating coils were glowing a somberly deep red. Along the walls were the switchboards, dozens of them, all sizes, all types of instruments, strange to the eyes of the terrestrians, and in practically all the light-beam indicator system was used, no metallic pointers, but tiny mirrors directing a very fine line of brilliant light acted as a needle. The system thus had practically no inertia. "Are these the changers?" asked Arcot gazing at the gigantic tubes. "They are; each tube will handle up to a hundred thousand volts," said Stel Felso Theu. "But I fear, Stel Felso Theu, that these tubes will carry power only one way; that is, it would be impossible for power to be pumped from here into the power house, though the process can be reversed," pointed out Arcot. "Radio tubes work only one way, which is why they can act as rectifiers. The same was true of these tubes. They could carry power one way only." "True, of tubes in general," replied the Talsonian, "and I see by that that you know the entire theory of our tubes, which is rather abstruse." "We use them on the ship, in special form," interrupted Arcot. "Then I will only say that the college here has a very complete electric power plant of its own. On special occasions, the power generated here is needed by the city, and so we arranged the tubes with switches which could reverse the flow. At present they are operating to pour power into the city. "If your ship can generate such tremendous power, I suspect that it would be wiser to eliminate the tubes from the circuit, for they put certain restrictions on the line. The main power plant in the city has tube banks capable of handling anything the line would. I suggest that your voltage be set at the maximum that the line will carry without breakdown, and the amperage can be made as high as possible without heat loss." "Good enough. The line to the city power will stand what pressure?" "It is good for the maximum of these tubes," replied the Talsonian. "Then get into communication with the city plant and tell them to prepare for every work-unit they can carry. I'll get the generator." Arcot turned, and flew on his power suit to the ship.