CH. XIII] RHEOSTATS AND OTHER BALLAST 525 



In wiring the rheostat, it is to be placed in one wire, (in series) as 

 all the current must pass through it (fig. 188, 281). It makes no 

 difference whether it is placed in the wire going to the upper carbon 

 or coming from the lower carbon. 



729. Calibration of a rheostat. The maker of a rheostat 

 should mark plainly upon it its capacity if it is of the fixed form. 

 If it is adjustable, then the range of the rheostat should be given. 



Furthermore, the lower range should be plainly marked at the 

 lowest step and the highest range at the highest step. The user of 

 a rheostat like that in fig. 145 could not tell easily which way to 

 turn the knob to increase or diminish the current unless the maker 

 indicates the amperage at the two ends of the steps. In case there 

 is no indication a person can determine it for himself if he has an 

 ammeter. 



Insert the ammeter in one wire of the line (fig. 273). Turn the 

 knob of the rheostat to the middle step, insert proper carbons in the 

 arc lamp, and turn on the current. When the lamp is burning 

 properly note the reading on the ammeter. Turn the knob toward 

 one side and the ammeter will indicate whether there is more or less 

 current. One can in this way find the amount of current delivered 

 for the different positions. It is well to mark on the rheostat face 

 with white paint the amperages corresponding to these positions. 

 It is also a help to have an arrow pointing from the lowest to the 

 highest amperage (fig. 182, 281). 



730. Home-made rheostats. While it is altogether false 

 economy to use anything but the best in the form of a rheostat it 

 is worth while knowing how one could be made in case of urgent 

 need. 



731. Barrel or bucket type of salt water rheostat. A wooden 

 bucket or barrel is used. In the bottom is placed a large plate of 

 iron, and one end of the supply wire is firmly fixed to this. The 

 other end of the wire is fixed to another mass of iron. The barrel 

 or bucket is then filled nearly full of water, and enough common 

 salt added to make about a ^4% solution. The water should be 

 well stirred to evenly distribute the salt. The upper iron and 



