Manual for submarine mining

by United States. War Department

3. Testing the battery and the apparatus for grounds with the galvanometer.—With a camel’s-hair brush go over all the instruments and carefully remove dust. See that the instruments and connections are dry. Do not blow on the instruments.

Open the battery switch. Connect the battery leads to the battery switch. Disconnect lead PX at P and connect the earth leads BY and EY to the key at “cable post.” (Y is grounded.) Both battery leads are left connected to the key. The shunt should be on 0. Close the battery switch. Close the testing key to the right. Turn the shunt gradually to the unity post. The galvanometer deflection should be zero. Turn the shunt to 0. Reverse the testing key. Turn the shunt to the unity post. The deflection should be zero. If any deflection is obtained, there is a ground in the battery, the apparatus, or the connections. The test of the cable should not proceed if a deflection is obtained in either position of the key.

In reporting the voltage + to earth and - to earth as “zero” on form, it will be understood that this means zero using the galvanometer, as herein described.

4. Insulation of leads.—Turn the shunt to 0. Open the battery switch. Connect the earth leads BY and EY to their proper posts. Connect the cable lead, PX, to “cable” post. See that the cable tank ends of the lead PX is disconnected at X and suspended in the air. Close the battery switch. Close the key and turn the shunt to the unity post. Deflections should be as small as possible and in any case must be steady and uniform for several trials. Turn the shunt to 0. Reverse the key, stopping at the discharge position. Turn the shunt to the unity post and wait until the galvanometer rests at 0, indicating that the leads are discharged. Turn the shunt to 0. Close the key all the way down. Turn the shunt to the unity post. The deflection should not differ materially from that noted above. If there is a deflection, the trouble is in the lead PX or its connections. Go over these, carefully examining for dust and moisture and noting particularly the proximity of all wires of opposite potential which cross or lie near each other. If there is a small deflection which can not be removed, a correction must be applied subsequently to the deflection obtained in the test for the insulation resistance of the conductor.

Using proper care, there are very few days when perfect insulation of the instruments can not be secured. The lead leakage with well-insulated wire put up properly will be noticed rarely.

5. Use of Price guard-wire.—As an additional precaution against surface leakage across the insulation at the ends of the conductor it will sometimes be advisable to install an additional lead (not necessarily as carefully insulated as PX) running from the testing switch to the cable under test. This lead should be connected in at the testing switch to the post carrying the lower blade between “D” and “C” ([fig. 16]); the tank end should be bare of insulation for a sufficient distance to enable the bare wire to be wrapped firmly, without pinching, around the insulation at each end of the particular conductor under test, just below the tapered portion.

The potential difference between the cable core and this guard-wire is thus made practically nil, so that any leakage will be from the guard-wire to the tank, consequently this leakage will not be measured by the galvanometer.

IV. Take the galvanometer constant as follows: Open the battery switch.

With a short piece of wire connect the hinge post of the testing key marked “cable” to either “earth” post of the key, the leads to the cable tank being disconnected at E, B, and P. Turn the shunt to 0. Examine the ⅒ megohm box and see that all the resistance coils are in the circuit. Close the battery switch and the testing key. Turn the shunt to the ¹/₁₀₀₀ post. Watch the swing of the galvanometer and when it has come to rest, read and record. Turn the shunt to 0. The galvanometer should return exactly to 0. If it does not, readjust and repeat until it does. The galvanometer constant is numerically equal to the total throw in smallest divisions of the scale multiplied by 100. Remove the connecting wire and replace the leads to the tank.

If at any subsequent time during the test the galvanometer adjustment is disturbed—that is, if it does not return accurately to zero when the shunt is at 0—the constant should be redetermined.