Abstract: After being baked out to remove absorbed gases, fursed silica cell blanks are cooled from a temperature of around 900.degree. C. to a temperature of around 800.degree. C. over a period of at least two hours. After the blanks are cooled to room temperature, an unsaturated vapor of atoms having an S.sub.o ground state is driven into the cell blanks from a reservoir. The cell blanks and the vapor contained therein are subjected to optical resonance radiation for around four hours at room temperature while the cell blanks are connected to the reservoir. A prescribed vapor density is established in the cell blanks before they are sealed off from the reservoir.

Abstract: A method and apparatus for making measurements of an electrically conductive material, which measurements can be used to determine the separate parameters thickness and conductivity without knowledge of the value of either of these parameters and in a manner giving improved freedom from sheet flutter, in which a transmitter coil is placed on one side of the sheet of material to be measured and a receiver coil on the other side at fixed distance from the transmitter coil and the transmitter coil is operated at two separate frequencies to obtain two relative phase relationships between the current in the transmitter coil and the electromotive force in the receiver coil such that the resulting phases differ in their functional relationships to thickness and conductivity, the frequency and phase being measured to obtain values which then can be used in independent equations to solve for at least one of thickness and conductivity.