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of Mu-metal and copper. As Mu-metal has an enormous permeability these tubes form an excellent magnetic shield. This shielding is necessary not only to remove the field of the earth but also to screen out all a. c. magnetic fields. In order to minimize the virtual vacuum leaks arising from the space between the concentric cylinders of the shield, radial holes were drilled through the cylinders, (although in different locations in each cylinder in order to eliminate magnetic "leaks"). The cavity region is shielded with soft iron cylinders, not shown. In use these cylinders are demagnetized by applying a strong 60 cycle a.a. magnetic field.

The entire structure of the vacuum envelope is supported on the vacuum header which is bolted to a brass plate which in turn is bolted to a wooden framework as shown in Figure 7. Tests have shown that the geometrical alignment of the apparatus is satisfactory with no additional support.

2.2 The Cavity Section.

The cavity is supported inside of a large brass ring (see figure 8) which also serves as a header for cavity cooling water and is drilled to provide a vacuum pumping area around the cavity. The cavity itself is made in three pieces, two end plates and a spacing ring. These pieces must be optically flat in order that the cavity length can be accurately determined. It is planned that this measurement will be made by interferometric methods. It must also be possible to remove some 200 watts by water cooling. The end plates of the cavity were made from $1⁄2$" copper plate and electroplated on one side with an .020" layer of chromium. Chromium has the same electrical conductivity as