II4 C. A. HEILAND 
provided for beth the milliammeter and the voltmeter, and as non- 
polarized electrodes are provided (reference list No. Ie). 
d. MODIFIED GISH-ROONEY APPARATUS 
A number of oil and mining companies have designed their own 
Gish-Rooney equipment, which differs slightly from the original in 
one way or another. The Michigan School of Mines has obtained 
much experience in the design of this equipment, and their apparatus 

Fic. 10b.—Construction of commutator in instrument shown in Figure roa. 
was used as model for one of the instruments built in the instrument 
shop of the Colorado School of Mines (Fig. 10 a and Fig. 10 8). 
Figure 11 shows the latest form of a Gish-Rooney equipment 
built at the Colorado School of Mines. On the left side of the figure, a 
box is shown containing the potentiometer, milliammeter, switches 
and jacks, and below the panel, the commutator, which is driven by 
a small D.C. motor, operated from a storage battery. The battery 
box is seen between the reels and the meter box. 
é€. MEGOHMERS 
These instruments have been used for years for the testing of 
grounds and insulation in electrical power-plant-, railway-, and radio- 
444 
