Manual for submarine mining

by United States. War Department

3. Set up the galvanometer on a pier or on a window sill if the building is of masonry. It should be insulated by placing its feet on a slate or ebonite slab, or in glass insulators. Remove the cover. Adjust the level until the suspended coil hangs freely. Maneuver the suspended coil, by means of the knob at the top of the tube, until its face is parallel with the face of the instrument. Then adjust the level until the upper suspension hangs in the center of the supporting tube, and the air gap between the coil and armature is symmetrical. Replace the cover. Put on the scale and the telescope. Turn the mirror so that it reflects the 0 of the scale approximately, getting exact adjustment by moving the scale. Be careful (particularly in dry weather) not to touch the glass of the cover or to do anything which will produce a static charge on the glass.

The galvanometer scales are usually graduated in equal divisions corresponding to 1 millimeter on the circumference of a circle whose radius is 1 meter. Each tenth division is usually marked with a number. This number is sometimes 1 instead of 10, 2 instead of 20, and so on. The number of divisions to read and record is the number of smallest (millimeter) divisions. Do not try to read closer than ½ of one division. The larger the throw the less the personal error. No accurate conclusion can be drawn from a very small throw.

4. Place a table or low shelf conveniently to one side and place the shunt, the testing key, the ⅒ megohm box, and a voltmeter on it. The apparatus should be insulated by an ebonite or slate slab, or glass insulators. Fasten the shunt and the key securely to the table or the shelf. (The use of paraffin paper for insulating instruments is a makeshift at best. It soon gets soiled and creased, then it has to be replaced.)

The use of lamps to keep the apparatus dry may be desirable, or it may be found convenient to expose the apparatus to the sun for a few minutes before beginning the test on any day. The use in the testing room of a small stove or of a gasoline torch for two or three hours before the beginning of the testing will ordinarily prove very advantageous.

5. Wire up as in [figure 16], except that the leads from the testing key should be carried to the battery through the double-pole single-throw switch above referred to. (The battery switch should be opened whenever any connections are made or altered.) All leads used in connecting up the instruments should be of heavy copper, and stiff enough to hold permanently any shape to which they are bent. They should be supported at points of connection only, and should not lie on the table or within an inch of each other.

III. Testing the insulation of the apparatus.—1. Voltmeter test of battery insulation.—This is a rough test, but should be included. A serious ground can be much more quickly located with a voltmeter than with the galvanometer.

(a) Disconnect the battery leads at the battery switch; connect + lead of battery to + post of the voltmeter; connect the B end of the lead BY to - post of the voltmeter; - lead of the battery should be in the air. Close the voltmeter switch and read.

(b) Disconnect the voltmeter. Connect - lead of the battery to - post of the voltmeter. Connect the B end of the lead BY to + post of the voltmeter; + lead of the battery should be in the air. Close the voltmeter switch and read.

If any deflection is obtained in either case, the battery or its connections are grounded. Locate and remove the ground. (See Foster or some other practical handbook.)

2. Testing the battery voltage.—Connect the voltmeter across the battery terminals. Read and record the voltage. (If there is no voltmeter available which will read as high as the battery voltage, take the voltage of the battery in sections and add, or make a multiplier of one of the resistance coils in the ⅒ megohm box.)