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MINUTES OF THE CHICAGO ELECTRO-MEDICAL

SOCIETY.

The December meeting of the society was held in room 412 Atlas building December 22 at 8:30 p. m. Dr. Burdick as president was in the chair. The committee on membership gave a report of progress and a general discussion on methods of arousing interest in electro-therapeutics and the X-ray ensued. The following were proposed for membership: E. T. Rellahan and N. O. Nelson. The election of officers was deferred for 3 months. A paper was then read by Mr. C. H. Treadwell on "Explanation of the Generation of Charges in the Static Machine." The paper was discussed by Drs. Grubbe and Burdick, Mr. Nelson and Mr. McIntosh. It was agreed that the discussion of the paper be continued at the next meeting.

PHYSICAL PRINCIPALS

INVOLVED IN THE

OPERATION OF THE STATIC MACHINE.*

BY C. H. TREADWELL, B. S.

Mr. President and Members of the Society:

I shall attempt to set forth the principles on which the static machine operates, confining the discussion entirely to the Holtz and the Toepler-Holtz machines, the Wimshurst machine operating on a slightly different principle. It must also be stated that this exposition is not at all original, for I shall endeavor simply to give what I understand are the views of physicists on this subject.

*Read before the Chicago Electro Medical Society, Dec. 22, 1903.

The static machine should be called an induction machine, and it is therefore necessary to explain the production of inductive charges under conditions as simple as possible. I will therefore perform a few introductory experiments with these insulated electrodes of the static machine, using these because of the simplicity of the operation and because any physician can repeat the experiment.

This hard rubber rod I will suspend at its center so that it may swing horizontally. It will be charged negatively when rubbed with this piece of flannel (see Fig. 1). We see that the insulating handle of this electrode (see Fig. 2) is also charged negatively and that there is repulsion between the two. This illustrates the general law that like charges (two negative charges or two positive charges) will repel, whereas unlike charges will attract.

We will now generate a positive charge by induction upon this large ball spark electrode, which serves as an insulated conductor. You see it is necessary for me to hold the charged hard rubber rod—that is, the inductor-above the insulated conductor and then touch this last body with the finger (see Fig. 3). The finger is then withdrawn and the inductor afterward removed and the body is now charged. By testing its charge upon the suspended hard rubber rod (see Fig. 4) we see that there is attraction, and we therefore conclude that this body is charged positively. This is charging by induction and we see the steps of the process; first, a charger inductor; second, an insulated body; third, the inductor brought near the insulated body; fourth, connection made with the earth, which is then broken, and afterward the inductor is removed.

It might be interesting to ask where this charge comes from. Some might say that there is a sort of circuit produced through the body of the experimenter, but that supposition is at once overthrown when the operation is repeated with the experimenter on an insulated support and some one else touching the body. Here there is no circuit in the usual sense of the word. Again it may be said that the charge comes from the earth or from the surrounding atmosphere or from the inductor itself. This last supposition may be dismissed when

we consider that the induction is charged negative, while the insulated body becomes positive. How could a positive charge come directly from a negative body? To test the adequacy of the other two suppositions we now perform this following experiment:

The large ball electrode is held gently against the roller electrode, thus forming two insulated bodies in contact. The charged inductor is now brought parallel to the ball electrode and the roller electrode is then separated from the ball elec-. trode (see Fig. 5). We have now induced charges upon these two bodies and by testing with the horizontally charged rod we see that the ball electrode attracts and the roller electrode repels. Whence came these charges? Evidently not from the ground, because there is no earth connection. Evidently not from the air, else why should one be positive and one be negative? The easiest way of explaining this phenomenon and one accurate enough for our purposes is to say that a neutral conductor has both positive and negative charges of equal amount. The negative inductor attracts the positive charge upon the adjacent surface of the ball electrode, and repels the negative charge to the roller electrode.

We will now return to the preceding experiments where the ball electrode is charged positively (see Fig. 3). When the negative inductor was held over the ball electrode the positive charge was drawn to its upper surface and its negative charge repelled to its lower surface. When earth connection was made the negative charge was repelled into the earth, the positive charge being held by the negative charge of the inductor. The earth connection being broken and the inductor removed, a free positive charge was left on the ball electrode. The method of induction therefore consists in bringing an inductor into the neighborhood of a neutral insulated conductor, earth connection being made and then broken, the inductor finally removed, leaving the insulated body with a free charge. This operation must be shown to take place on the static machine. We need first, however, to apparently modify the experiment—namely, if we put two glass plates

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between the inductor and insulated conductor, the operation of induction will still produce charges.

We shall consider this little model because there is no case around it and we can touch all its parts. It is identical in principle with the large Toepler-Holtz machine (see Fig. 6). The smaller circle represents a rotating plate on which are glued the metal disks. The larger circle is a stationary plate on which are the paper sections shown in Fig. 7.

Corresponding to the finger is this brass rod called the neutralizing rod (see Fig. 6a). These metal disks on the rotating plate are the insulated conductors, while the inductors are the paper sectors on the stationary plate (see Fig. 7, I). These are sometimes called condensors or armatures, or the field, but I have much doubt of the suitableness of the term. They can scientifically be called inductors. One of these inductors becomes charged positive and the other negative by friction between the tinsel brushes on the arms fixed to the inductors, or, if you please, by the contact charge due to the touching of these different metals in the tinsel brush and the disks on the rotating plate. However, if only a very feeble charge is given to one of the inductors, the machine will generate. Referring to the experiment on induction, we see that the important time in the process is when the finger touches the insulated conductor, and so it is when the metal disks on the rotating plate pass beneath the tinsel brush on the neutralizing rod that charges are generated. Let this left inductor be positive and the disk be in position indicated. by the figure. (See Fig. 6.)

Note.

The plus signs on figure IV should be on the metal part, not on the insulated handle. In figure V the ball electrode should be positive and the roller electrode negative.

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