increase of current, so that a further growth of current is efficiently checked. It is in this way that the working of the Nernst lamp is made stable.
The ballast resistance is made of fine iron wire; and if this were allowed to become nearly red-hot whilst exposed to the air, it would very soon burn out. It is therefore necessary to protect this delicate spiral of wire from the air, and this is done by enclosing it in a sealed glass tube. This tube is filled with hydrogen, since hydrogen has, of all gases which could be used in this case, the greatest heat capacity. It would obviously be a mistake to use an exhausted tube as a protecting envelope for the iron spiral, since through a vacuum very little heat can be transmitted, and it is obviously important to prevent the spiral from getting more than dull red-hot, otherwise it would be destroyed. If, then, a gaseous filling is indispensable for the conveyance of the heat generated in the spiral to the outside envelope, we must use a gas which will not burn the iron. Air is therefore inadmissible. Nitrogen or carbonic acid might be used, but these gases do not convey heat so readily as hydrogen, the lightest of all gases, and whose molecules are the most
