a density of 0 7 g cm 3 thickness 50 u03bc and contains 0 02 weight of boron with respect to absolutely dry material the same material is used for preparing by the same process and on the same equipment but with the use of desalted water control samples of electric insulation paper the density and thickness of which equal those of the paper containing boric acid the value of tan u03b4 is determined for the samples of paper of both types in the following way paper samples are collected into a pack 280 320 u03bc thick the pack is pressed between flat electrodes 46 mm in diameter under a pressure of 0 2 kgf cm 2 and dried in vacuum under a residual pressure of 5 10 4 mm hg at 125 u00b0 c for 2 hours then the packs are cooled in vacuum under the same residual pressure down to experimental temperature which is 30 u00b0 60 u00b0 100 u00b0 or 120 u00b0 c and thermostatted at one of these temperature value after the electrodes are connected to the measuring arm of the schering bridge the tests are conducted with the use of industrial alternating current with a frequency of 50 hz at a voltage of 5 v u03bc the tan u03b4 values are being read directly off the schering bridge after the latter has been balanced table 1 presents the values of tan u03b4 determined for the paper samples obtained according to the described process table 1______________________________________ tan u03b4 of the samplestempera containingture u00b0 c boron control______________________________________ 30 0 00105 0 00105 60 0 00085 0 00085100 0 00085 0 00085120 0 0010 0 0012______________________________________ electric insulation paper is prepared under laboratory conditions on the basis of pure electric insulation cellulose obtained.