The S4 Electromagnetic Current Meter measures the voltage resulting from the motion of a conductor (water flow velocity) through a magnetic field according to Faraday's law of electromagnetic induction. Simply stated, Faraday's law defines the voltage produced in a conductor as the product of the speed of the conductor (water flow velocity) times the magnitude of the magnetic field times the length of the conductor. In the case of the S4, the conductor length is the effective path between the sensing electrodes. The magnetic field intensity is generated by a circular coil, internal to the S4, driven by a precisely regulated alternating current. The use of an alternating magnetic field and synchronous detection techniques to measure the voltage at the sensing electrodes provides an extremely stable, low noise current measurement. Two orthogonal pairs of electrodes and an internal flux gate compass provide the current vector.

The S4 Solid-State Memory
Data obtained by the S4 is stored internally in solid-state, highly reliable, non-volatile memory. From this memory data is retrieved through an RS-232-C port to the user's terminal, computer or other storage device without the need to open the instrument housing. This eliminates the need for unreliable tape readers or removable memory cartridges. Turnaround time for data retrieval is short so instruments may be redeployed almost immediately.

The S4 is Software Controlled
The EPROM-formatted microprocessor affords unprecedented flexibility and simplicity of use. The low power CMOS microprocessor of the S4 performs true vector averaging, burst sampling and adaptive sampling. The instrument can alter its recording format in response to oceanographic events. Customized programs can be developed to meet the special needs of researchers.

The S4 is equipped with extensive internal software which permits the operator to select many operational features such as intermittent or continuous operation, time on, time off, numbers of samples for vector averaging, high speed, high resolution and adaptive sampling.

The S4 is Solidly Built
All electronics and power necessary for operation of the current meter are contained within the compact 10 inch (25 cm) diameter sphere. This sphere is made of a durable, high strength, dimensionally stable, corrosion proof plastic. There is nothing to break or foul. Connection to a mooring is by means of an axial titanium load bearing shaft. The only other metal parts in contact with water are the titanium electrodes. This combination of materials provides the user with a rugged, easy to use current meter that can take extended deployment in harsh seawater environments, without fear of corrosion or bio- logical attack. Additionally, the chances of handling damage are minimized.