Article by John Higgins
Many pinball machines make use of electro-magnets to enhance gameplay and I, for one, am a big fan of these features.
Unfortunately, it is well known that pinballs themselves become magnetised.
I had noticed balls getting stuck in the ball trough, not loading into an upkicker or not sitting directly on trough ball microswitches/optos, causing the machine to think a ball was missing.
When it happened, I would open the coin door to see the balls all bunched together in the trough, but not rolling down.
My first thoughts were that the ball trough itself had become ”pitted”. I also doubted the angle of the ball trough for a moment, as a slight adjustment allowed the balls to roll down normally. It then became apparent that the balls were magnetised and “pulling“ together stronger than the force of gravity, or simply sticking to the metal ball trough.
This did not happen for every game; only when certain balls landed in the trough next to other magnetised balls and also when they randomly lined up with magnetic polarity matching or opposing. This would cause the pinballs to not line up with the microswitches in the ball trough, as they were pushing the top ball back up the trough by about a centimetre.
Over the years I have amassed a selection of pinballs which look in reasonable physical condition but I cannot trust not to be heavily magnetised.
The problem - how to find out which balls were magnetised and by what strength of magnetic flux.
A magnetic field can be represented by lines of induction or flux lines/dipoles. These lines are invisible and are produced by magnetized material or by electrical currents.
Magnetic flux measured in Gauss (G) – It is named after Carl Friedrich Gauss, an early researcher in the field of magnetism. Magnetic objects are surrounded by a magnetic field. Some devices can detect this field and also give information about the direction/polarity of the field and even its strength.
• Deflection of a compass needle – most metal items will cause this effect, not very precise.
• Pick up small pins – again, not very precise or scientific.
• Show on magnetic flux paper/pole sensor foil – gives a good visual indication of magnetised pinballs, although not very precise (see the picture & the video below).
• Polarity indicator test magnet – only indicates polarity on pinballs that are quite heavily magnetised, i.e. ~30 gauss (sourced from GuysMagnets.com).
• Iron filings – good fun but messy.
• Electronic indication from a gaussmeter – gives an electronic reading that equates to an actual magnetic force in Gauss. Also indicates north & south polarity, which is difficult otherwise with a spherical pinball. Very precise although home made units may not be calibrated.
Gaussmeters are used to measure the strength and polarity of a magnetic field. They use a electronic chip called a Hall effect device, which gives off a tiny electrical current when exposed to a magnetic field. The current is amplified with electronic circuitry and a meter shows the number of gauss (the units of magnetic field strength).
To find out how strong a magnet - or the strength of the magnetic field of a particular pinball - really is, you can purchase a Gaussmeter (also known as a magnetometer). These can be pretty difficult to find and relatively expensive, although they are precise and calibrated.
I built a hand-held Gaussmeter for measuring the polarity and strength of a magnetic field. It uses a Hall effect device and some op-amps from RS Components.
Here’s a schematic of a Gaussmeter circuit (using a 3503 Hall-Effect Device):
I built two versions of these gaussmeters, each with different Hall effect ICs. The first was more sensitive than the other and the second one was designed to measure larger gauss readings.
One Gaussmeter showed a difference in measured voltage of 7.5 millivolts per gauss, with a range of 400 gauss. The other displayed a difference of 25 millivolts per gauss and is therefore more sensitive to smaller magnetic forces. It was this version that I chose to use in a hand held gaussmeter
To make a hand held pinball gaussmeter I built a variation of this circuit, due to the fact that the Hall effect IC I got from RS operates on an optimum voltage of over 8 volts, not 5 volts.
So, a 7809, 9v voltage regulator was used. A more sensitive Hall effect IC was used, reading a deflection of 25 millivolts per gauss. (RS Components Hall effect sensor – part number: SS94A1F) and filtering capacitors were added to smooth the signals.
This unit to be powered by a 9V battery, housed in a small project case. The hall effect IC can then sit under a near-pinball sized hole in the case. This makes the unit easy to rotate the pinball to find optimum magnetism strength and polarity, whilst also being portable and more sensitive to very low magnetic readings (e.g. pinballs).
To calculate the Gauss reading using a gaussmeter with a 25mV hall effect sensor
V0 = Initial voltage reading of hall effect IC with no magnet at the sensor
Polarity can also be determined using the gaussmeter. If the digital volt meter reading increases as the magnetic source approaches, the sensor is facing the north pole. If the DVM reading decreases the sensor is facing the south pole.
Observations made using the home made Gaussmeter:
I was unable to track down technical specification for power of the game electro-magnets (e.g. the Moon magnet in Apollo 13 or the crane magnet on Judge Dredd), although measuring with gaussmeter gave the following readings:
These measurements are not as exact as other readings due to the fact that the machines blows the magnet fuse if held for too long (e.g. while measuring for magnetic flux strength).
It is generally assumed that carbon core pinballs are less susceptible to magnetism than solid steel pinballs. I would like to experiment further with this before commenting.
General susceptibility to magnetic forces are difficult to quantify, due to the following reasons:
Many people will have the knowledge of this subject to calculate and elaborate. However, I do not have the expertise in physics to calculate these diversities through mathematics.
Pinballs do have the advantage of rarely landing in front of the machines electromagnet with the same orientation. Otherwise, magnetism would occur much more rapidly than it does.
A brand new pinball measured 1.2 gauss.
The pinball left attached to large magnet (210 gauss) for 24 hours: now 6.4 gauss
Pinball attached for another 24hrs, same orientation: now 9.2 gauss
Conclusion – The new pinball readily gains magnetic properties
I would estimate a lot of gameplay would be needed to match that of 48hrs constant exposure to a magnetising force. For example, Apollo13 grabs the ball for an average of 4 seconds. This would equate to 960 ball grabs to magnetise a new pinball to ~10 gauss.
Heating: Heating of a magnet to a temperature above what is known as the “Curie point” will allow the molecules/ magnetic dipoles to realign and thus become demagnetised.
This is not practical for pinball as the Curie point of most steels is ~600C, with little or no effect taking place until around 300C.
These temperatures would cause physical damage to the surface of a pinball.
Hammering/Dropping: Repeated hammering or dropping on a hard floor will also cause enough vibration to randomise the molecules/magnetic dipoles, and thus reduce the magnetic flux.
An experiment indicated that dropping a magnetised pinball (40 gauss) 50 times from a height of 2m onto a concrete floor caused little or no change in the measured magnetic flux. Yes, I actually did this and yes, this also considerably ruined the physical surface of the pinball.
Degausser: The method of removing/reducing magnetic fields using an AC electrical unit is referred to as “Degaussing”.
A degausser uses a rapidly alternating magnetic field to neutralise an object's existing magnetic field.
Due to “magnetic hysteresis” it is generally not possible to reduce a magnetic field completely to zero, so degaussing typically induces a very small “known” field referred to as "bias".
Experiments with a hand held degausser
I was able to experiment with a hand-held degausser – the type used to degauss a CRT monitor. This unit had an “effective magnet” of 100G at 1cm.
It was found that the degausser was quite effective in neutralising the magnetic properties of a pinball measuring 10 gauss or less through 20 x 5 second applications. A pinball measuring 9.6 gauss would measure 1.6 gauss after degaussing by hand in this way.
However, it was also found that this unit had little or no effect on pinballs measuring over 20 gauss.
I have read this is due to the fact it requires a magnetic force 7 times that of the original to achieve demagnetisation. Therefore, a more powerful degausser may be able to demagnetise a heavily affected pinball.
NB: I would point out that overuse of a degausser leads to overheating of the unit and it could become a fire hazard.
My personal observations and conclusions; most of which are pretty obvious.
Some pinballs are becoming heavily magnetised – up to half that of standard ferrite fridge/project magnets.
Grabbing machine magnets (e.g. Judge Dredd/Apollo 13) will magnetise balls quicker than those which “fling” the ball around (e.g. Iron Man/Spiderman).
Pinballs with a magnetic flux reading of ~20 gauss and over tend to create intermittent gameplay problems.
Pinballs with magnetic flux reading of ~40 gauss and over tend to give constant gameplay problems.
A non-metallic ball trough would help in this regard.
In-trough degaussing may be an option for some machines, but would be an expensive production outlay and also possibly giving rise to other electrical problems or even fire hazards.
Trying to demagnetise balls is pretty much a waste of time. If they are heavily magnetised, it is too time consuming and destructive, depending on method used.
The magnetic flux film is a pretty easy way to visually determine which balls are worst affected.
I would also like to emphasise that this experiment has been documented purely from a hobbyist point of view and I am in no way a Physicist or magnetism expert. The field of magnetism is an extremely intricate, interesting and complicated subject with many variables.
Please note that information has been taken from freely available sites on the internet. Credits to the following sites which you should visit for further/better information: