Portable L. Drault Induction Coil f/ X rays


This coil was obtained from France in May 2007.


The coil was apparently in Paris during WWI and was removed to prevent the German occupation from taking it.


This was a comment supplied by the originator of the coil:

Well, the coil was a part of a small laboratory in Paris in 13e district, and which was moved to south of the capital in anticipation of the German invasion in 1914 at the beginning of the WWI.”



The coil was used for X rays and was made by L. Drault. Numerous pieces are marked by the manufacture, the coil top, meters, mercury interrupter armature and top and the phenolic back and bottom block of the meter post.



The coil is unique in that it was designed with a mercury interrupter as standard. The interrupter was driven by the electromagnetic filed generated by the primary coil of the coil.


The rectangular piece of metal seen behind the pot is the core of the coil. The rotor of the interrupter is in alignment with this metal.

















The interrupter has an internal rotor that has two simple spiral tubes inside it. When rotating, this pumps the mercury up and out the 2 jets. The 2 streams of mercury make contact with 4 arced pieces of metal mounted underneath the top of the interrupter. This effectively shorts across the 2 arms on the top of the interrupter. The mercury allows high currents and quick breaks for a very rapid coil charging.

The interrupter gives 4 breaks per revolution corresponding to each arm position of the external armature.



The long knob locks the pot in a certain position. The pot was rotated to vary the speed of the interrupter. It did this by changing the timing of the voltage applied to the coil in relation to the rotor position. The external swastika shaped armature is timed to the internal rotor by a set screw in the armature and a hole in the rotor shaft.

Lubrication of the rotor shaft is accomplished by a small oil hole thru the hub of the external rotor which directs the oil to the shaft bearing mounted in the insulated top.


Situated inside the wooden mounting base of the interrupter is a condenser connected across the interrupter.

2 binding posts are also mounted to the interrupter contacts which serve to connect an external interrupter if desired.





The 2 pet cocks on the interrupter were used to purge the internals of the pot with coal gas. This was to reduce oxidation of the mercury and components. It was very important to make sure all air was displaced with gas before operating the turbine or it could explode! Although the gas was flammable, if there was no oxygen to sustain combustion, the gas was inert.


















Most portable coils were very simple in that they had a pair of binding posts to connect the battery. This coil has a complete control panel comprised of an ammeter, 0-15 ADC, and a voltmeter, 0-30 VDC, a simple lead wire fuse block and an ON/OFF switch.

The ammeter is always connected to the coil and the voltmeter has a push button to temporarily monitor the battery voltage. The voltmeter has a very low resistance and would drain the battery if left connected, hence the push button to momentarily place the meter in the circuit.



The ON/Off switch is interesting in that it is also where the power is applied to the system. The rectangular slot on top of the switch has a pair of contacts situated across from each other where the batter power would come in.

The switch has a red line on one side of the slot to indicate the polarity side of the battery. If the battery is connected with the + corresponding the red line, the polarity is correct for the coil and meters. This would make the HV connections of the coil always be the same polarity as well and simplifies connecting the X-ray tube.


The power plug was also the key to turn the coil on. The original plug was missing so a new plug/ key was fabricated trying to keep the look with the period of the coil.

















The HV posts for the coil are very interesting in themselves. The bases of both posts are slotted to lock them in a fixed orientation on the coil top.

The meter post has a milli-ammeter incorporated in it. The meter was used to monitor tube current and it is in series with the coil. The meter range is 0-3 ma.



There is a large hole in the meter post and it is directly in line with a point on the other post. It is assumed there would have been a spark gap on the coil as this was a common on X ray coils.

The gap was set a certain distance to determine the “hardness” of the X ray tube. When the tube was operating, the gap was set to a certain distance for the particular X ray work to be done and to determine the hardness or vacuum level of the tube. For example, most work was typically 3” of spark.

When the coil was running, the spark gap was adjusted until a spark would pass across the gap. This distance was what the tube would “back up”. By knowing this distance, the proper tube could be selected for the desired operation.

A spark rod was made for this post.


























The second post is very interesting. There is a series spark gap mounted on this post. All connections seem to have been made after this gap.

Series gaps were used on induction coils to block inverse currents when the interrupter was first energized. This inverse was of opposite polarity and much lower voltage for the X ray tube and can shorten the life of the tube or even damage it if the inverse was high enough. The gap was set to prevent the lower voltage from jumping the gap and being applied to the tube and would fire when the correct and higher voltage was produced from the coil.


This apparent series gap is adjustable and is enclosed in a glass cylinder with 2 connections. The exact purpose of the connections is not known yet and it is assumed an inert gas was circulated thru the tube to reduce oxidation of the metal when the gap was operational. So far, I cannot locate any references to this particular arrangement.




Of interest, the posts could not be stored in the coil and it is amazing they remained with the coil all these years.



One would be hard pressed to truly call this a “portable” coil. It measures 6 ½” wide by 22 ¼” long and 10 ¼” tall and weighs 57 pounds without the HV posts and 60 pounds with the posts!


A suitable battery would have weighed at least 30 pounds. The total outfit with a tube and stand was probably approaching 100 pounds!
















Operation of the coil would have been something like this:


The coil was opened and set up with the battery wires connected to the plug and the X ray tube connected to the appropriate posts of the coil.


The plug was inserted in the switch and the pushbutton checked to verify the battery had a good charge.


The switch was turned on, at this point, the coil is ready to run but there is no current flowing thru the coil yet.


The red knob on top of the interrupter is given a quick twist clockwise to pump some mercury and start the coil. If the twist is fast enough, the coil will start and the interrupter will run and come up to speed of approximately 700-800 RPM with a quite bump, bump, bump.. noise. This is equivalent to 2800-3200 breaks per minute for the coil.


The spark gap can be set to verify the resistance of the tube or tubes so the proper tube can be selected for the desired X ray. At the same time, the tube can be viewed with a hand held fluoroscopic screen to verify it is operating properly.

The switch was turned off to stop the operation.


Once the system was verified, the patient was placed under the X ray tube and the desired exposure was made.

Exposure times could take up to an hour back then and a stop watch was a necessary accessory to the Dr.’s X ray routine.



This is a rare find and a thrill to see it operational once again.


Frank Jones 11 June, 2007