Two electrical essays: The black box

AN ELECTRICAL ENGINEER is asked to investigate a setup as shown in Figure 1. A d-c generator with a terminal voltage of 100 volts furnishes current to a series combination of resistance of 5 ohms and a black box with two terminals. The current in the circuit, as indicated by the meter M1 is 10 amperes, and the voltages across the two series elements are each 50 volts, as indicated by the meters M2 and M3. The 5-ohm resistor is rated 500 watts, but it runs a good deal hotter than the manufacturer claims it should with rated wattage; the black box, on the other hand, runs completely cool. The engineer explains this latter phenomenon by proclaiming that electric energy is not converted into heat, but into some other form of energy in the black box (for instance it may contain batteries being charged). Just to be sure of himself he connects a wattmeter in the circuit. The current coil of the wattmeter is placed in series with the ammeter, and the voltage coil is connected successively across the generator terminals, the 5-ohm resistor, and the black box. The readings of the wattmeter in the three places are respectively as follows: 1,000 watts, 1,000 watts, and zero watts for the black box!