42 THE DESIGN OF STATIC TRANSFORMERS 



l>y different manufacturers. While the film-of-oxido 

 method leads to a higher iron factor, /./., while the loss of span- 

 due to insulation is less than in either of the other two methods, 

 it is none too reliable. Papering the plates on one side has been 

 asserted to be the most reliable method, but it entails considerable 

 expense whether carried out by machinery or by hand. Paper 

 insulation also takes up somewhat more space than does a film 

 of oxide or of varnish, and is not likely to be as permanent an 

 insulation as that provided by a suitable varnish. The paper 

 will ultimately be reduced to powder under the influence of the 

 alternate heating and cooling of the core. In some cases of 

 paper-insulated cores which were taken apart after a few years 

 of service, the paper was found to be in an advanced stage of 

 disintegration, and the laminations had become oxidised under 

 the influence of exposure to certain constituents of the paste 

 with which the paper had been applied. Tlie danger of 

 disintegration is, however, lessened in oil-cooled transformers 

 as the oil tends to preserve "the paper. The thickness of the 

 core plate is determined upon from a study of the component 

 losses. It is evident from consideration of the results 

 embodied in the curves of Figs. 17 and 18 (on p. 38), that 

 with the modern alloyed iron, the eddy loss is so exceedingly 

 low that far less advantage is obtained by employing thin sheets 

 than was the case with the formerly-used quality which, as has 

 been pointed out, had very low specific resistance. Whereas 

 with the former material it was important, in high-periodicity 

 transformers, to employ sheets of as low a thickness as 0,35 mm, 

 it is my opinion that with the newer material, whose specific 

 resistance is three times as great, there is insufficient object in 

 employing a less thickness than 0,50 mm ; in fact, I should be 

 inclined to adopt this or even a greater thickness, as a convenient 

 standard for transformer cores. It must be remembered that the 

 thicker the transformer plate, the less is the percentage of space 

 occupied by the insulating varnish on the plate. Taking 

 0,025 mm as a representative thickness of core-plate varnish, 



