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Magnetic compatibility, blackberries and apples

A few years ago, I had a run of bad luck with paper rail tickets that would be rejected by automated ticket barriers, leading to the embarrassment of reversing out of line and then finding a ticket inspector to let me through. My luck seemed no better with hotel keys that would often not be recognised. Eventually, I worked out that it was my mobile phone (Blackberry) that had a strong permanent magnet on its case flap. This was used to trigger a Hall effect magnetic sensor device in the Blackberry to tell it that the case had closed.

The tickets and hotel room key cards used magnetic stripes that got wiped when the magnet in the Blackberry case was getting too close in my pocket. Two incompatible systems that had not been designed to coexist.

I was interested to hear recent reports of Apple devices affecting the operation of pacemakers. Similar to the Blackberry case including a magnet, the iPhone now includes permanent magnets to add useful features. Some magnets are in an arrangement to physically align charging cables with an inductive charging antenna for enhanced power transfer. Other devices such as Fitbit watches use magnets for the alignment of power cables to make physical connections.

Pacemaker devices have a trick of using the presence of a magnet field to control an aspect of the operation of the device. “Magnet-mode” is typically used to switch a sophisticated pacer device into a simple, safe mode of operation when required. The static fields from a magnet penetrate the body and allow an implanted device to sense the field from the magnet and then change its behaviour in magnet-mode. Apple’s advice is to keep your iPhone twelve inches away from nearby medical devices that can be affected by the permanent magnet fields. The FDA recommends keeping consumer electronic devices that may create magnetic interference 6 inches away from implanted medical devices. Good advice – but how practical is that as a way to avoid the incompatibility and risk of interference? How many users will forget this and put their iPhone in a shirt pocket too close to an implanted medical device?

Some pacemakers use non-static, but very low frequency magnetic fields to communicate more than a message to control Magnet mode. One system uses “8K telemetry”, using a wand antenna to establish an 8 thousand bits per second link with a monitoring device to allow the pacemaker to report on its performance, its battery level and download records of how the patient is responding.

At this low frequency, the magnetic field penetrates deeply into the tissue and is sensed by the implanted pacemaker device which can establish a connection to transfer information by this magnetic communications channel. The 8kHz bit rate is chosen so that the frequency band used in the communication is lower than the 9kHz where the regulation of electromagnetic fields starts. So, this communication is “under the radar” in this low frequency, unregulated and inductive communications band.

Fortunately, this is a low range system. The wand antenna has to be impressed on the body close to the site of the implanted antenna. This mitigates the risk of connecting the wand to the wrong pacer by mistake. (There is a famous scene in the drama “Homeland” where a pacemaker device is hacked and remotely controlled. This is fanciful, at least with inductive communications because scenarios of connection and control of a pacer over metres of range are not very realistic with this type of link.)

The use of static fields to communicate presence, to control antenna position or to communicate are all very useful applications, but there is little regulation over these low frequency magnetic fields to ensure non-interference between different systems. Because most low frequency fields are short range these systems do not interfere, but as we have seen there is increasing use of static magnets because of the useful properties of very low frequency magnetic fields. So we can expect to hear similar examples to the new iPhone of interference between systems.

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