contract. These Daleks have a silver colour scheme with blue/grey shoulders, blue hemispheres and eye discs, collars in natural aluminium and black fenders. The story refers to them being powered by static electricity, with mobility limited to the metal floors in the Dalek City on their home planet, Skaro. In the episode "The Ambush" a Dalek is shown using a device resembling a thermal lance to cut through a door. It consists of a long, slim tube flanked by two globes, surmounted by a clear disc-shaped "sight screen". This appendage is mounted to the Dalek's arm in place of a plunger. The Dalek concept was the creation of screenwriter Terry Nation, with the design being realised by BBC staff designer Raymond Cusick. The props were constructed by Shawcraft Engineering of Uxbridge, England under the direction of Bill Roberts, who also provided additional design input. Four 1960s style Dalek props were recreated by Titan Props for the British television docudrama An Adventure in Space and Time (2013), commissioned by the BBC to celebrate the Doctor Who programme's fiftieth anniversary. Mark 2 Daleks The storyline for the second serial to feature the Daleks, The Dalek Invasion of Earth (1964), required the props to be used extensively on location. Consequently, serial designer Spencer Chapman and manufacturer Shawcraft Engineering devised a tricycle arrangement, incorporating three pneumatic tyres, to replace the original castors and carry the props over uneven ground. Enlarged fenders were then created to hide the revised undercarriage. To explain the Daleks' ability to travel away from the static-charged floors of their city, the narrative has the Doctor stating that an aerial located at the edge of a mine in Bedfordshire is the key to the Daleks' power supply on Earth. Although not referred to directly in the story, it is implied that the parabolic dishes now fitted to the rear shoulder section of each prop act as receptors for a form of transmitted energy. Other design changes are a reduction in the number of eye discs to five and painting some eyeballs silver instead of the standard black colour. An amphibious capability is demonstrated in the closing moments of the first episode of the serial when a Dalek emerges from beneath the waters of the River Thames. For the first time a Dalek command structure is introduced, with rank indicated by differing colour schemes. The Earth Task Force Commander, or "Saucer Commander",