© Brooke Clarke 2013

Horse Power
Permanent Magnets
Measuring Magnetic Fields


After studying DC permanent magnet motors I'm looking into making my own motor.  One way to do that is to combine permanent magnets with electromagnets.  So I'm starting this page to look into both type of magnet.

When talking about magnetic field strength, it's helpful to compare to the Earth's field, see my sensors page for more on that.

Horse Power  (Wiki)

If you wanted to make a 1 horse power DC permanent magnet motor for a bicycle what would be required?
HP = 32572 Foot pounds of force per minute. (see Wiki for HP)

A bicycle going at different speeds will have some revolutions per minute. Assume rolling radius of tire is 13" (26" road tire), Circumference of tire is 81.7" or 6.8'.

Speed Miles per Hour
Linear Speed Feet/min
=5280 ft/mi / 60 min/hr * Mi/hr
1760 2640
Rotational speed RPM
= Linear speed/6.8'
Pounds of force needed at 1 foot radius for 1 H.P.
=32572 / RPM
Force needed on one face of permanent magnet
Pounds of force needed along circumference / 4

If the permanent magnets are arranged around the rear wheel so that the polarity alternates (N, S, N, S, ...) Then an electromagnet can be attracting one pm while repelling another thus doubling the force.  In a like manner if the electromagnet in wound on an inverted "U" shape then each end of the pm will be either attracted or repelled thus cutting the force needed in half again.

Permanent Magnets (Wiki)

The Rare Earth magnets (Wiki) are currently the strongest yet economically practical so that's what I'm going to be looking at.  The Neodymium (Wiki) is the one most commonly used.
One source for these is K&J Magnetics, they have a calculators that will tell you about Pull Force, Repelling Force, Magnetic Field, Plate Thickness & Field in a gap with or without a yoke.
There are Wiki pages for: Magnetic Force,
Ampère's force law, Magnetic Constant (Vacuum permeability µ0),
Note:  I would expect that the magnetic field between a couple of magnets (wither pm or em) would increase if a yoke connected the back of each.  This becomes more and more important as the gap approaches zero length.

Older permanent magnets needed to be long and skinny in order to have strength.  You could think of a short and fat magnet as made up of a bunch of long skinny magnets tied into a bundle.  But the modern rare Earth magnets can be very strong even though they are short, like a coin with the poles as the heats and tails.

Electromagnets (Wiki)

One of the first uses for electromagnets was for telegraphy and self winding clocks (1 & 2), doorbells, phone ringers, etc..  In these applications two side by side electromagnets had their cores connected together by an iron strap leaving two pole pieces exposed to attract an armature.  These typically had soft iron cores, it was only later that Silicon steel laminations were used to reduce losses.

The key equation is B = u0 * k * I * (N/L)
B = field (Tesla)
u0 = 4π×10−7 (N·A−2)
I = Current (Amps)
N/L = Turns per meter.

The N/L term may assume a single layer coil construction rather than a multiple layer coil.  That's because the first layer encloses some area, but the next layer encloses a larger area and so on for more layers where each successive layer generates a weaker and weaker field.

Using a smaller wire diameter makes for a stronger field (N/L) increases, but smaller wires can carry less current.  The total length of wire and so the voltage drop across the coil (and the power dissipated in the coil) are related to the diameter of the coil, so a smaller diameter is desirable, i.e. only as big as needed.

Measuring Magnetic Fields

There are a large number of methods of measuring magnetic field strength.  There are AC and DC methods, but only DC measurements will work for permanent magnets and a static Earth measurement. 
The measurement sensors may have an offset that needs to be adjusted and that may require a zero field test chamber.  See: Sensors
It's possible to use a rotating coil or moving a PM through a coil to measure it using a Fluxmeter.


Unfortunately the situation with magnetic units is similar to RS-232 serial communications where there is an agreement to not have a standard.
It appears the same agreement has been made when it comes to magnetic units, everyone chooses a unit that they like.   For example here are some conversions:
1 Oersted (Wiki) = 79.58 Ampere-Turns/Meter
1 nano Tesla (Wiki)= 10 micro Oe = 1 Gamma
1 Tesla = 10,000 Gauss
1 Maxwell (Wiki) = 1 gauss × cm2 = 10−8 Weber (Wiki)

Also see: Magnetism (Wiki), Magnetic circuit (Wiki),


DC Permanent Magnet Motors

Walker Scientific Inc. MG-3D Gaussmeter (93F Hall Effect probe)

The MG-3D is a digital display instrument with five full scale ranges from 10 gauss to 100,000 gauss for AC or DC fields, displayed on a ±0.05% 3-1/2 digit bipolar DPM offering ±0.1% resolution.

5 Full scale; 10, 100, 1K, 10K, 100K with 100% over-range

TYPE-Digital; ±0.05% plus 1 digit 3 1/2 digit bipolar DPM

RANGE-0.01 to 199.9

NORMAL MODE - For measurement of DC (steady state) fields only.

RMS MODE - Meter displays the RMS value of the measured field (AC and DC components). Accuracy is ±1% for measurement of fields from 3Hz to 10Hz, ±0.3% 10Hz to 400Hz within frequency response stated.

PEAK READING OPTION - ±0.1% of full-scale range within stated frequency response

Analog Output
ANALOG VOLTAGE - ±1 volt full-scale Overrange capability to ±10 volts without loss of accuracy. Output is proportional to the measured field.

ACCURACY - ±0.1% or 10 milligauss, whichever is greater

FREQUENCY RESPONSE - DC to 400 Hz (3 dB down at 400Hz)


LOAD IMPEDANCE - Minimum of 2,000 ohms; short circuit protected

Power Input
Approximately 6 Watts; .06 amps @ 105V to 125V or .03 amps @ 210V to 250V - 50Hz to 400Hz

STANDARD - 115V 50Hz to 400Hz
Helmholtz Coil
    Sensitive Research Instrument Co. Fluxmeter
    Annis M25 Pocket Magnetometer

HP 428 Clip-on Milli-ammeter - with both clip-on probes and a magnetometer

AlphaLab DC Gaussmeter Model 1
    AMY6 Magnetic Polarity Tester
    GE Gauss Meter & Reference Magnet
Sensors - Magnetic - Helmholtz Coil
Telephone - Telephone patents
Electromagnetic devices:
Electromagnetic Toy Engine
Gilbert DC 3-pole Electro-magnetic Machine
Leclanché Battery - wet cell
MESCO 1011 Toy Engine
No. 6 Dry Cell -
Toy Motor Kit & modern version as well as Science First demonstration motor -
Vibrators -
Weeden DC 2-pole Electro-magnetic Machine - really a generator
Magnets - this web page
Sonobuoys & CRT-1 Sonobuoy
DC Permenant Magnet Motors
Walker Scientific MG-3D Gaussmeter
Wireless Driveway Monitor - solar magnetometer


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page created 23 Sep 2013