Section 60. The punishment of death shall be inflicted by causing a current of electricity of sufficient intensity to cause death to pass through the body of the prisoner, the application of said current to be continuous until he is dead or, at the election of the prisoner, by intravenous injection of a substance or substances in a lethal quantity sufficient to cause death and until such prisoner is dead. The sentence shall be executed within an enclosure or building for that purpose at the state prison and the company which furnishes the electric power or light to the state prison shall provide all necessary electricity for executions by electrocution at such times as the superintendent orders.