The large hole in the mine case covered by the cap is closed by a plug. The joint is made water-tight by a lead washer jammed between the plug proper and the case and by a coating of red lead or similar waterproofing material upon the screw threads. In the strong currents and deep water of some harbors more buoyancy than is possessed by the 32-inch case is required. This is obtained by inserting between the hemispheres a cylinder of 20-pound wrought iron which is stiffened by extra welded ribs for the larger sizes. Such cases are designated by the diameter in inches of a sphere having the same buoyancy. Thus, a No. 40 case is made by inserting a cylinder 32 inches in diameter and 20.4 inches in length between the two hemispheres of a No. 32 case; this is sufficient to make the displacement equal to that of a spherical case 40 inches in diameter. In the latest types the cylinders are made of corrugated mild steel of less thickness, which diminishes very materially the weights of the cases.
The following table exhibits the dimensions and weights of buoyant mines, with trotol fuse cans, complete except the charges and moorings. The actual free buoyancy when planted will be the difference between the displacement and weight as given in the table, reduced by the weight of the charge and of the moorings and cables:
| PLAIN CASES. | |||||
|---|---|---|---|---|---|
| No. | Displacement. | Computed weight, empty. | Measured weight, empty. | Length of cylinder. | Remarks. |
| Pounds | Pounds | Pounds | Feet | ||
| 32 | 635 | 308 | 311 | 0.00 | All are about 33½ inches in |
| outside diameter; the extreme | |||||
| length in each case is 4.3 feet | |||||
| plus the length | |||||
| 33 | 695 | 364 | .17 | ||
| 34 | 762 | 395 | .35 | ||
| 35 | 829 | 427 | .54 | ||
| 36 | 904 | 462 | .75 | ||
| 37 | 982 | 498 | .96 | ||
| 38 | 1,064 | 538 | 1.20 | ||
| 39 | 1,149 | 578 | 1.43 | ||
| 40 | 1,242 | 621 | 625 | 1.70 | |
| 41 | 1,341 | 665 | 1.96 | ||
| 42 | 1,436 | 712 | 2.24 | ||
| 43 | 1,540 | 788 | 759 | 2.53 | |
| 44 | 1,652 | 842 | 2.77 | One extra welded rib. | |
| 45 | 1,767 | 876 | 3.17 | Do. | |
| 46 | 1,887 | 952 | 3.50 | Lot of 1879; | |
| one extra welded rib. | |||||
| 899 | |||||
| 936 | Lot of 1884; | ||||
| one extra welded rib. | |||||
| 47 | 2,013 | 1,011 | 3.85 | One extra welded rib. | |
| 48 | 2,144 | 1,073 | 1,037 | 4.20 | Do. |
| CORRUGATED CASES. | |||||
| 47 | 1,536 | 572 | 2.24 | ||
| 50 | 2,323.2 | 777 | 4.22 | ||
The compound plug, with old model brass fuse can.—A section of this plug, with the names of all the parts, is shown in [figure 5]. The brass fuse can is not used when guncotton is used as a priming charge.
The compound plug, with rubber fuse can.—A section of this plug, with the names of all the parts, is shown in [figure 6].
The compound plug, with trotol fuse can.—A section of this plug, with the names of all the parts, is shown in [figure 7].
In each plug the main parts are screwed together and held in place by set-screws. The connection of the compound plug with the mine case makes an earth plate, of which the electrical resistance in salt water is about 1 ohm.
The mine transformer ([see fig. 8]).—This consists of a cylindrical brass case, which contains the primary and secondary coils of the transformer and the reactance coil. The transformer is screwed into the brass collar or the reenforce and in turn has the circuit closer screwed upon its top. The fuses are attached to the secondary and are fired when proper voltage is applied to the primary. The primary leads are black; those of the secondary are red. The terminal, P′, of the primary coil is left free for the purpose of testing, but when preparing the transformer for use it is attached securely to the binding post, T. The upper terminal, R′, of the reactance is prepared for attachment to the ball seat of the circuit closer.
The normal circuit is from P, through the primary coil (the resistance of which is about 2,400 ohms), to the transformer case, and thence to earth. However, when the mine is struck, so as to close the circuit closer, a parallel circuit is closed through the reactance (the resistance of which is about 130 ohms), thence to the ball seat of the circuit closer, through the ball and springs to the transformer case, and thence to earth. In this latter case, therefore, the resistance is lessened by about 2,300 ohms.
The reactance coil will permit only a small amount of alternating current to pass through it when the ball is displaced, hence mines may be fired whether the ball is displaced or not.