The Heavy Element Engine was the main power source utilized by the Martians during the Invasion.
It's description is as follows:
One of the more exciting discoveries prompted by investigation of Martian technology is the source of much of the power for their various Machines. These consist of rods or bars of heavy metals contained in a case treated with a similar energy-blocking substance to the gravity shield arrays of the Flying Machine. This is with good reason, as the heavy elements used to power the Martian Machines emit a dangerous amount of Becquerel rays. The Martians were apparently aware of the dangerous nature of their power sources, as the case for the Heavy Element Engine is extremely sturdily constructed, in several cases being the only part of a destroyed fighting machine to remain intact. This is a fortunate thing for the inhabitants of the Earth, for if even one of these engines had been broken open, the area surrounding it would surely have become uninhabitable due to the deadly radiations released.
The mechanism by which these radiations were converted to usable energy is very elegant. A coolant fluid was used to transfer the heat from the Heavy Element Engine to an electrically insulated chamber of viridigen gas. The heating of this gas in a confined space would cause it to release a negative charge, which would be channeled away by means of electrodes in the walls of the chamber. By using a series of these chambers, and allowing them to heat and cool in turn, a steady flow of electricity could be generated.
Study of the Heavy Element Engine has been hampered by the fact that, with few exceptions, when the Martians realised at the end that they were dying they shut down their engines permanently. Apparently built into the engines as a failsafe were a number of meatal rods which were extracted or inserted to control the output of the Engine. The complete insertion of these rods served to shut down the Engine almost completely, and the procedure cannot be easily reversed. Research into the Heavy Element Engine has thus been suspended until a method can be developed to remove the rods while still allowing some control over the output of the Engine - it has been theorised that complete removal of the rods could cause an Engine to produce a catastrophic amount of heat.
According to the said report, the enormous energy output of the Engines also made it very hazardous due to the deadly radiations produced.