THE electromagnet designed for the use of oculists by Dr. Haab, director of the Zurich eye clinic, and shown in the accompanying photographs, is the most powerful and efficient apparatus of this kind in existence. According to Dr. Haab, the principal requirements of such an instrument are the following: 1. The magnet must be capable of developing very great tractive force. 2. It must be placed horizontally, and its circuit must be made and broken by the operation of a pedal. 3. The working pole must have a form adapted to its peculiar function and its dimensions must bear the proper reIation to those of the coil. The first condition is very important. The small electromagnets which are commonly used by oculists often fail at the critical moment, owing to their lack of adequate magnetic strength, and thus endanger the success of the operation. The method of making and breaking the circuit by the foot of the operator possesses great advantages, because it leaves both hands free for the control of the eye and body of the patient, and makes it unnecessary to remove the magnet in order to stop its action. This last point is exceedingly important for, even with the most perfect suspension, it would be impossible to withdraw the heavy electromagnet from the eye quickly enough to prevent the iron filing or other foreign substance from being drawn into a region where its presence is not desired. When the circuit is opened and closed by a pedal the magnet need not be suspended. Dr. Haab regards the suspension method not only as impracticable for the heavy magnets which he employs, but as incorrect in principle. His experience proves that it suffices to support the magnet in such a manner that its axis can be turned to any direction in a horizontal plane. In regard to the form of the working pole, it is obvious that its action will (je weakened in proportion to the extent to which it protrudes from the coil. It must not, however, be so short that the coil interferes with the view of the field of operation, as is the case in some of the newer large electromagnets. The magnet is of the bell form. The working pole if; a cone of GO degrees, and this end of the coil is tapered sufficiently to give the operator a clear view of the field up to the tip of the magnet pole. Four pointed tips accompany each apparatus. The idle pole is expanded into the form of a bell which covers and protects the greater part of the coil. This construction possesses the additional advantage of restricting the escape of lines of force to one side and diminishing the area of the external magnetic field.
This article was originally published with the title "Powerful Electromagnet for the Use of Oculists" in Scientific American 105, 26, 573 (December 1911)