THE MEASUREMENT OF RESISTANCE 



205 



not to the tank plate. As the tank is grounded, any leakage cur- 

 rent due to imperfect insulation of battery and leads will be 

 measured by the galvanometer, if it be connected to the tank 

 plate, and the results of the test vitiated. 



Any current which leaks over the surface of the insulation from 

 the projecting wire of the cable to the tank will cause error in 

 the results. This leakage current must be reduced to zero. 

 Therefore any protective covering, such as braid or armor, 

 should be removed from the ends of the wire for a distance of 

 at least 18 in., thus laying bare the insulating coating. This 

 latter should not be handled and must be kept scrupulously 

 clean. As an additional safeguard, the insulation should be 

 cut back from the end of the wire, as indicated in Fig. 113. 

 This must be done with a sharp, clean 

 knife, so as to leave a perfectly clean 

 surface of considerable length (2 or 

 3 in.). In order to be absolutely 

 certain that all surface leakage has 

 been eliminated, recourse should be 

 had to Price's guard wire. This de- 

 vice is shown in Fig. 115. 



A few turns of bare and flexible wire 

 are closely wrapped around the insu- 

 lation a few inches from its end so that they make perfect con- 

 tact with it; the wire is then carried to the battery side of 

 the galvanometer. The potential difference between the core 

 and the guard wire is practically nil, so that any leakage will 

 be from the guard wire to the tank; consequently the leakage 

 current will not be measured by the galvanometer. The guard 

 wire may be used for a variety of purposes; for instance, to pro- 

 tect the galvanometer lead if it be long and cannot be made an 

 air line, or to protect a part or all of the testing apparatus. 



The shunt S, the commutator Ci, the galvanometer G, the 

 resistance R, and the key KI must be thoroughly insulated. 

 They should all be mounted on posts of polished hard rubber at 

 least 4 in. high, and may be protected by a guard wire. The 

 lead from KI to the cable should, if possible, be an air line. 



Even after all precautions have been taken, it will not do to 

 assume that leakage is not present. A test must be made to 



FIG. 115. Diagram for 

 Price guard wire. 



