94 Rood — Experiments on High Electrical Resistance. 



pended in the air by insulating threads. With a set of con- 

 densers like the above, the capacity of any condenser within 

 their range can be quickly obtained. 



Units of resistance. — After a number of experiments in 

 different directions, it was decided to employ peroxide of 

 manganese painted on strips of blue cobalt glass, which was 

 found to be a much better insulator than colorless plate glass. 

 The mode of construction, briefly, is as follows : washed com- 

 mercial peroxide of manganese is painted with a brush on 

 cobalt glass, the edges of the strip being avoided, and it is then 

 dried with gentle heat. The layer thus obtained is quite 

 adherent in spite of the fact that only water is used in making 

 the mixture, and it can be laid on with different thicknesses 

 and have desirable breadths. The two ends of the glass slip 

 are then wrapt with tin-foil, and about 7 cm of the foil allowed 

 to remain free at the two ends to hold the unit when it is 

 immersed in a rosin-wax bath. Fine brass wire is used to pre- 

 vent the unit from unrolling itself when it is lifted up. The 

 rosin-wax bath contains only so much wax as does not dim the 

 polish of the finished unit, the polish of the bath itself, when 

 it cools, being about half destroyed. Pure yellow wax was 

 employed, and the temperature of the bath was about 150° C. 

 Immersion in this bath drives out air and moisture, and as 

 more than half of the tin-foil is also immersed, it results that 

 the unit is embedded in the insulating substance, conduction 

 to its interior being furnished by the free tin-foil at the ends. 

 Afterwards, while still as hot as allowed handling, the units 

 were finished with a hot knife, care being taken repeatedly to 

 press down the foil at the places of contact. The superfluous 

 rosin being removed, the remainder of the tin-foil was wrapt 

 around the ends of the units and suitable terminals of sheet 

 brass supplied. These again were wound with flexible iron 

 wire having a diameter of 0'4 mm . In this operation as much 

 force as was considered safe was employed, in order to secure 

 fixity, and to make sure that the pressure exerted on the foil 

 should be far greater than any experienced afterwards in the 

 use of holders to. connect the units, or to introduce them into 

 the circuit. It will be seen that the units are completely inac- 

 cessible to moisture, and experiments have shown that in ordi- 

 nary weather their surface conduction, practically, is zero. 

 Three-quarters of the insulating surface between the terminals 

 have been wrapt with tin-foil without lowering the resistance, 

 even when the hygrometer indicated 60 per cent of moisture. 



The process which has been explained is suitable for the 

 production of units having resistances from one up to ten 

 thousand megohms. For higher units the oxide of manganese 



