392 PROCEEDINGS OF THE AMERICAN ACADEMY. 



soned wood of a given kiod, but only in the sense that wood of resist- 

 ance as high as this can always be obtained without difficulty in the 

 market. Abnormal specimens occur. In measuring the insulation 

 resistances between the copper terminals of un paraffined maple and 

 birch spools, like or similar to the one represented in Figure 1, I ex- 

 perimented upon a large number of spools which had been lying for 

 about a year in a certain dry closet. The smallest resistance in any 

 case was 1,100 megohms, and the average resistance was more than 

 2,000 megohms. This is what one may expect to get in spools of this 

 kind. In the same closet were some spools of a different lot, bought 

 of the maker of the other spools, and in no way different in appear- 

 ance from them. These also had been seasoning beside the others for 

 a year, yet the average insulation resistance of these spools was only 

 a little over one megohm. This is an extreme case. The nearest 

 approach to it that I found in experimenting on other lots of spools 

 was that of some which had been standing for a long time in the 

 damp basement of the laboratory, and represent what the ordinarily 

 good dry spool might become if it were placed for months in a moist 

 place. Yet the lowest resistance in the case of these spools was more 

 than 100 megohms. 



It is, of course, well known that the insulation resistance of a po- 

 rous half-conductor depends very much upon the amount of moisture 

 which it contains, and that this moisture may give rise to all manner 

 of anomalies, as Du Moncel has shown. Thus, white marble when 

 it comes from the mill is often a fairly good conductor, owing to the 

 water which it has absorbed in the process of manufacture, but a fort- 

 night's drying in the sun sometimes increases its resistance ten thou- 

 sand fold. It is now almost always possible to get kiln-dried wood, 

 and after wood or marble has once been thoroughly dried, an immer- 

 sion in a bath of hot paraffine tends to prevent the reabsorption 

 of moisture. Red vulcanized fibre absorbs hot paraffine greedily ; but 

 I do not think that it would be easy to saturate a piece of fibre so 

 thoroughly with paraffine that a drop of water allowed to rest on its 

 surface for a few moments would not begin to raise a blister. 



Prolonged immersion in clean, hot, melted paraffine always in- 

 creases the insulation of a half-conductor, even if the bath leaves no 

 perceptible coating on the outside. This increase, however, is very 

 slight in the cases of some close-grained substances like rosewood, 

 though it may amount to three or four times the original resistance in 

 the case of a porous conductor. If while such a conductor is immersed 

 in hot paraffine the bath and its contents be placed under the receiver 



