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 |
CHAPTER II.
CONTACT BREAKERS.
Fig. 18.
The simple form of contact breaker already described is useful up to a certain point, but it has disadvantages. Its rate of vibration is only variable through narrow limits, and it is not suitable for very heavy currents. But as it stands it has done long service, and will be used probably wherever the requirements from it are not exacting. The most desirable form of this simple spring break is shown in Fig. 18. R is the soft iron armature; S, the spring; C, check-nut which holds the adjusting screw A from becoming loose; T, a second adjusting screw used to tighten the spring and so raise its rate of vibration; K is the base to which one wire of the coil is attached; L, base of adjusting device to which battery wire runs at I. Where tightening screw T passes through the pillar of the adjusting screw, the hole therein is bushed with rubber to prevent accidental contact. Both A and T are provided with insulating heads of rubber or ivory. At B are the platinum contacts, which should be fully ⅛ inch in diameter.
One serious defect in the action of the simple spring vibrator (Fig. 19) is the tendency of the spring to vibrate, as it were, sinusoidally. This causes an irregularity in the rate of the vibrations, which affects the discharge of the coil very considerably. By far the better plan is to use a very short thick spring riveted to an arm carrying the armature at its end (Fig. 20). R is the armature, S the piece of spring, and K the point of attachment to the base. The actual width of the portion of the spring which vibrates—the hinge portion, it might be called—should not be over ⅛ inch.
Fig. 19. Fig. 20.
The rate of motion is high; but an erroneous notion has been taken of its performance by many persons in the knowledge of the writer. The rate of vibration is not wholly dependent on the size, or, rather, smallness of its spring; the arm and armature considerably alter this, although they are not pliable, by reason of their mass and the momentum consequent on their mass.
A word here on the size of the armature. It should be somewhat larger than the face of the electro-magnet core, and should be thick—that is, in a circular form—say one half its diameter. Of course this does not apply to the steel lever armature before mentioned. It is impossible to lay down arbitrary rules where the conditions are not determined, but a very small amount of experimenting will demonstrate the correct lines on which to build.
When in action, all rapid rheotomes give out a definite musical note whereby the rate of vibration can be determined. Reference to any work on acoustics will show a table of the number of vibrations necessary to produce any stated musical note. The foregoing style of rheotome forms the basis of very nearly all those which are in use. The shorter and stouter a spring the more rapidly will it vibrate, and vice-versa. Carrying out this rule, we can manufacture an instrument which will give as high as 2500 vibrations per second (Fig. 21).
Fig. 21.
The armature A is a piece of flat hard steel bar ¼ × ½ inch, held rigidly on the metal support S and just clearing the upper surfaces of the magnet cores C. The adjusting screw P should be provided with an arm, B B, whereby the rotation of it can be delicately varied. This screw must also be firmly held or the high speed of the armature will jar it loose. A check-nut on each side of the frame carrying it should be provided in every case. The necessary platinum contact can be hammered into a hole drilled before the armature is hardened. The proper place for this contact is about one fourth of the total length of the armature from its support, although in the simple contact breaker it can be placed at the distance of one third if desired. The reason is that the concussion of the adjusting screw dampens the free vibration, and the amplitude thereof is lessened in addition to the counter vibrations of the screw disturbing the regular vibrationary series.
Owing to the fact that the amplitude of the armature vibration is so small, a very delicate adjustment is necessary. The adjusting screw can be placed nearer the free end, but for the reasons given it is not to be desired. The metal bridge should be a solid casting, and the armature clamped by more than one screw.
The mercury vibrator, which is applied to almost every large coil, is as follows:
A pivoted arm carries on one end a soft iron armature, which is attracted by the coil core. The other end is provided with a platinum point adjustable by a set screw. This platinum point dips into a mercury cup—a glass cup containing mercury, with a thin layer of spirits of turpentine. The object of the spirits of turpentine, which is a non-conductor, is to help choke off the spark which would ensue whenever the platinum point was raised from the mercury.
A form of contact breaker which will admit of great variation of speed, and which is adapted to carry large currents, is the wheel-break, constructed in the following manner:
A brass or copper disk 3 inches or more in diameter and upward of ½ inch thick has its periphery divided by a number of saw cuts, which divisions are often filled in with plugs of hard rubber or fibre. This disk is mounted on a shaft, which latter is either the shaft of an electro-motor, or is provided with a pulley by which it can be rapidly rotated. A strip of spring copper on each side of the disk presses upon the toothed surface, one strip being connected to the coil and the other to the battery or other current source. It will now be seen that when the disk rotates the slits or pieces of hard rubber cause the break in the circuit through the brushes or copper strips, the rapidity of the breaks depending upon the rate of rotation of the disk, and the number of slits in the wheel.
The slits or rubber pieces should be one-half the width of the intervening brass, but must be at least one sixteenth of an inch in width, especially where a high voltage is used in the primary coil.
The shaft of the machine may serve as one point of connection in place of one of the copper brushes; but in this event either a wide journal must be used, or else some conducting substance, as plumbago, replace the lubricating oil in the bearings.