One of the Hearts of a TARDIS (like Time Lords a TARDIS has two) is a massive energy collector that is a mathematically constructed copy of the Prime Eye of Harmony on Gallifrey. In modern times it is often referred to as an Eye of Harmony (which can cause some confusion). These Eye copies can be found on all TARDISes after the Type 24. The Eye's iron-like Containment Sphere is located beneath the Cloister Room.While the Prime Eye's Containment Sphere is 10 km in diameter the TARDIS's sphere only about the size of a house.

Artron Energy created by the Prime Eye of Harmony on Gallifrey is broadcast into the Space-Time Vortex. This energy travels though the Vortex's Time Contours. Attached to a TARDIS's Containment Sphere is the Receptor Antenna, which collects artron energy using chronon transduction. The naked singularity at the Heart of a TARDIS is directly linked to the Space-Time Vortex, and uses Chronon Transduction to collect the energy.

This creates a link between a TARDIS's Eye and the Prime Eye on Gallifrey. The TARDIS's Heart then emits the beams of Artron Energy to power a TARDIS. The Eye is equipped with a Flux Comparator to ensure a steady flow of energy. The Eye also serves to slowly vent the potential energy stored from dematerialization. This Hawking Radiation is leaked from the Eye's event horizon. As long as the Prime Eye of Harmony on Gallifrey remains in existence, a TARDIS's energy source will never stop producing power.

The closer one gets to the Heart of the TARDIS, the stronger the dimensional "pressure" gets. This "pressure" can have minor effects on the nervous system. If someone were to actually look directly into the Heart, while it was transducting energy from the Prime Eye of Gallifrey, their consciousness would be destroyed by the intense blasts of Artron Energy, leaving only a mindless body behind. Like the other Heart of the TARDIS, the Main Space-Time Element the Eye can, under the right circumstances, restore the dead back to life. For more information see the entry on the Main Space-Time Element.

When a TARDIS is de-registered and made non-operational, the Eye is opened for a short time to de-energize it. Once de-energized, a TARDIS can only be reactivated using special Gallifreyan technology. A drained TARDIS can be "jump started" by inserting a charged Neutron Rod into the receptor socket on the Console. A Time Ring's power source can also be used. Whatever the source, the power reading must be well above .2 omegas for a TARDIS to be energized.

The Eye is protected from unauthorized tampering by a retina print lock that only allows Time Lords to open the Eye. This lock can only be changed by altering the Security Matrix on Gallifrey. If the Eye is opened outside of a Dry Dimension Dockyard, it will cut off the flow of Artron energy to the Dynamorphic Generators. Having the Eye of Harmony open alters the molecular structure of any nearby planets. The TARDIS will attempt to contain the Eye's energies but if it is left open, the containment fields will drain all of the Auxiliary Power within three hours. At the end of this time any nearby planets will be destroyed in a massive burst of Gravitational Energy. If the Eye is left open for too long the TARDIS will die. Stasis Halos (similar to the Sash of Rassilon) should be used by the operator whenever working with an open Eye.

There is a switch labeled "Do not push" that will collapse an entire TARDIS into the Eye of Harmony. A Time Lord who is very close to their TARDIS can probably send telepathic messages to cause its Eye of Harmony to detonate in a massive explosion that would destroy everything for miles. A TARDIS being destroyed in the Vortex might buckle causality in the Web of Time for centuries in either direction, and with the right technical knowledge and temporal abilities it would be possible to sabotage a TARDIS so that it would destroy the entire Universe.

Since the Heart of the TARDIS is a copy of the Eye of Harmony, it can also serve as an temporary independent power source if the Prime Eye of Harmony is out of range (see Noosphere Parameters) or has been disabled. However the fuel supply of this Copy Eye is not unlimited. Given that the iron sphere that holds the Prime Eye is about 70 million times as big as the one holding a TARDIS's Eye it is possible that the Heart of a TARDIS has only 1/70,000,000 the fuel capacity of the Prime Eye. A fully fueled TARDIS can travel at least 200 trillion years without completely exhausting the fuel supply. In any case, it appears that the Copy Eye is capable of supply independent power for at least a few years.

Without the Prime Eye of Harmony a TARDIS would eventually have to refuel itself. Fortunately the Eye's Receptor Antenna is also capable of collecting temporal energy from temporal instabilities, fault-lines running between dimensions, scars on reality, or rifts in time and space that found throughout this Universe. Time Rifts make the best fuel source. Such a rift occurs when two or more dimensions are pressed against each other creating a rift or a scar like an earthquake fault line. The TARDIS can drain off the energy created by the competing forces. Depending on the current energy level of the rift it can take a few minutes to a few days for the TARDIS to power up completely. The energy collected by the copy Eye is stored it in the TARDIS's other Heart (aka Main Space-Time Element) located in the Console. A TARDIS's Emergency Procedures will automatically try to divert any large explosion or energy release that occurs inside the TARDIS into its Eye of Harmony. It should be noted that fuel collected in this Universe can only be used in this Universe. Should a TARDIS that relies on scavenged fuel travel into a parallel universe it would be without power unless a source of Gallifreyan energy or a local temporal anomaly could be found. If any of the TARDIS's small Power Cells retain power, they can be charged up if the operator donates a dose of artron energy equivalent to 10 years of their life. Once the cell has finished its recharging cycle (about 24 hours) and powered back up, it can be used to give the TARDIS five minutes to attempt a space-time jump.

Should power from the TARDIS's Heart be unavailable, the TARDIS will have to rely on the Ancillary Power Station for emergency energy.