SHUDDEMAGEN. — RESIDUAL CHARGES IN DIELECTRICS. 473 



pasty liquid from under the tinfoil. Care had to be exercised in order 

 not to tear the tinfoil, which was very thin, — about 0.004 of a centi- 

 meter. It was bought in the form of a continuous roll, 30.5 cms. in 

 width ; thus only two sheets were necessary to cover each side of a 

 plate, a free margin of about 4.5 cms. being left around three edges, 

 while the tinfoil itself projected over the fourth edge about 1 cm. The 

 reason for using the paste instead of shellac was that paste is a con- 

 ducting material, and thin films of it, which might possibly be left over 

 the tinfoil, would not cause any residual charge, while if the dielectric 

 shellac had been used, these thin films might perhaps have given rise 

 to small but troublesome residual charges, which were especially to be 

 avoided. The tinfoils on the two sides of a plate projected over the 

 same edge of the plate, and were pressed down with thicker paste over 

 a fine strip of copper foil all along this edge. The copper wire termi- 

 nals of the condenser were soldered on to these strips with wax flux. 



To separate the tinfoil-coated glass plates, which must be done by 

 some very good insulator, it was decided to use thin glass disks, pro- 

 vided they could be found of the proper thickness, rather than disks of 

 hard rubber, because this latter substance changes its surface condition 

 in time. Fortunately a pane of glass was found of just the desired 

 thickness, 0.076 cm., and a great number of disks 1.1 cms. in diameter 

 were cut out of it and ground smooth at their edges. 



Ten of these were placed between every two successive plates of 

 glass, seven around the marginal space, and only three in the tin- 

 foiled region. For these three circular pieces of tinfoil, 2.5 cms. in 

 diameter were removed, and the paste below them carefully cleaned off. 

 The disks were pressed down onto the glass plate with a very small 

 drop of liquid shellac in between. Small weights were then placed on 

 top for a day or two, so that the shellac might have time to harden 

 under pressure. Then, for the sake of better insulation, a little melted 

 paraffin was guided around the under edge of each glass disk with a 

 hot iron wire. 



In the air condenser built up of these plates there were eleven layers 

 of air, each about three quarters of a millimeter thick. This condenser, 

 which was mounted in a large oak case made for the purpose, has a 

 capacity of 0.0428 microfarads and an insulation resistance of 35,000 

 megohms. 



The Falling Weight Machine. 



In studying the rate of formation of residual charges for very short 

 charging intervals, Professor B. O. Peirce's large falling weight machine 

 was found to be of the greatest use. A massive oak frame 244 centi- 



