Long ago in the days of cathode-ray technology there were two types of cathode ray tube. In the older technology, colour separation was achieved by using a shadow mask: a thin metal plate with a grid of holes that allow only the correct beam to strike a red, green or blue phosphor. A shadow mask can heat up and distort at high brightness levels ('doming'), causing the beams to miss their target and giving a blotchy image. To avoid this, shadow masks were made of Invar, a nickel alloy with a near-zero expansion coefficient between room temperature and \(150^{\circ} \mathrm{C}\). A consequence of shadow-mask technology was that the glass screen of the CRT curved inward on all four edges, increasing reflected glare.

Use the index you derive to rank the following candidate material for this application. carbon steel, stainless steel, nickel-chromium alloys and tungsten. All are available as fine wire.

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Function Constraints Objective Free variables Aperture grill for CRT Wire thickness and spacing specified Material must carry pretension without failure Electrically conducting to prevent charging Able to be drawn to wire Maximize permitted temperature rise without loss of tension Choice of material