A coil much used for electric baths has a primary winding only, regulated by the sliding in and out of the iron core, which necessitates the use of an independent vibrator, or else by varying the current strength with a rheostat. The general directions given before will answer in the present case, the only data necessary being the size of wire, which should be about six to ten layers of No. 20 B & S. The coil with movable secondaries here comes into service. Strong currents are needed for bath work, and any variety of winding can be used with this make of coil. There are so many descriptions of bath and small medical coils in the electrical magazines published for amateur workers, that it is hardly necessary here to give more than a mention of the principal ones.
Hints in Caring for Medical Coils.
A few remarks on medical coils and their diseases may not be amiss; often a very little defect, if remedied in time, will prevent costly repairs.
The main care in medical electrical apparatus is the battery (see Chapter X. for descriptions of coil batteries and their operation). Clean, fresh solutions and clean contacts are essential. Keep zincs well amalgamated, remove wires from binding posts, and scrape bright the metal where the wires make connection; see no fluid is splashed on contacts, clean all contact springs periodically. The Edison-Lalande battery is probably the best for medical use, but even this requires occasional attention as to contacts, new zincs, fresh solution, etc.
Poor adjustment at contact breaker, dirty or corroded contacts, loose wires, loose binding posts, corroded binding posts, are often the only trouble in a coil refusing to work.
Flexible cords are fruitful of trouble: the tinsel breaks, and there is no circuit; gets wet and crosses or causes a leak; cord tips get loose and alternately open and close a contact; one minute all is well, next minute no current can be obtained. Another trouble in acid batteries is caused by leaving the zincs in the fluid. It is easy to do it in most cases, although the ingenuity of the leading medical electrical apparatus makers to-day is directed to this point. Cleanliness and careful inspection of all contacts is well repaid; carelessness surely brings its evils.
It is very desirable in medical work to eliminate the noise attendant upon the working of the coil vibrator. This jarring or humming is often in itself a source of irritation to a nervous patient. The sound can be deadened in various ways, for instance, by placing over the vibrator a temporary wood cover, lined with felt, resting upon a soft rubber gasket; or in any other manner that may suggest itself to the operator.
Table Showing Resistances and Feet Per Pound of Copper and German Silver Wires.
| Gauge, Browne & Sharpe. | Diameter. | Feet per lb. | Copper. | German Silver. |
|---|---|---|---|---|
| Ohms per 1,000 ft. | ONLY APPROXIMATE | |||
| Ohms per 1,000 ft. | ||||
| 8 | .1285 | 20 | .62881 | 11.77 |
| 9 | .1144 | 25 | .79281 | 11.83 |
| 10 | .1019 | 32 | 1 | 18.72 |
| 11 | .09074 | 40 | 1.2607 | 25.59 |
| 12 | .08081 | 51 | 1.5898 | 29.75 |
| 13 | .07196 | 64 | 1.995 | 37.51 |
| 14 | .06408 | 81 | 2.504 | 47.30 |
| 15 | .05707 | 102 | 3.172 | 59.65 |
| 16 | .05082 | 129 | 4.001 | 75.22 |
| 17 | .04525 | 162 | 5.04 | 94.84 |
| 18 | .0403 | 204 | 6.36 | 119.61 |
| 19 | .03539 | 264 | 8.25 | 155.10 |
| 20 | .03196 | 325 | 10.12 | 190.18 |
| 21 | .02846 | 409 | 12.76 | 239.81 |
| 22 | .02535 | 517 | 16.25 | 302.38 |
| 23 | .02257 | 660 | 20.30 | 381.33 |
| 24 | .0201 | 823 | 25.60 | 480.83 |
| 25 | .0179 | 1039 | 32.20 | 606.31 |
| 26 | .01594 | 1310 | 40.70 | 764.59 |
| 27 | .01419 | 1650 | 51.30 | 964.13 |
| 28 | .01264 | 2082 | 64.80 | 1215.76 |
| 29 | .01126 | 2623 | 81.60 | 1533.06 |
| 30 | .01002 | 3311 | 103 | 1933.03 |
| 31 | .00893 | 4165 | 130 | 2437.23 |
| 32 | .00795 | 5263 | 164 | 3073.77 |
| 33 | .00708 | 6636 | 206 | 3875.61 |
| 34 | .0063 | 8381 | 260 | 4888.49 |
| 35 | .00561 | 10560 | 328 | 6163.97 |
| 36 | .005 | 13306 | 414 | 7770.81 |