Four-terminal Wires

Two routes for the synthesis of a four-terminal wire compound based on a central porphyrin unit are presented in Scheme 10.25 [13b], In one route, the tetrabro-mide 75 was treated with potassium thioacetate, which resulted in the substitution product 76. Alternatively, reaction of aldehyde 77 with pyrrole 78 at room temperature using mixed acid catalysis conditions provided 76 after oxidation with DDQ. 

Scheme 3.30 Synthesis of four-terminal wires 52 and 53, both with one methylene barrier.

Four-terminal conductivity measurements were made with gold wires attached to previously evaporated gold pads on the crystals with silver paste. Measuring currents (dc) were 0.1 - 100 pA,depending on sample size and conductivity behaviour. Hydrostatic pressure could be applied in a He-gas pressure cell. 

For measuring the conductivity and thermoelectric power, the molybdenum cell incorporates a 6-bore alumina tube leading from the cell into the cold closure structure via the capillary. Four bores contain molybdenum wires that serve as electrodes for a conventional four-terminal conductivity measurement with an external potentiometer. Two more wires consisting of platinum run from the center of the cell to the cold end of the alumina tube. These wires together with the sample form a mercury-platinum thermocouple the absolute thermoelectric power of mercury can be derived from the measured e.m.f. In the configuration illustrated in Fig. 7.2, the conductivity and thermoelectric power samples are located in the center of the density sample, and therefore the three quantities are measured on adjacent portions of the same bulk mercury sample. 

There are approximately 100 billion neurons in the human brain. Neurons have wire-like processes called axons, which transmit information away from the cell, and processes called dendrites, which receive information from other cells. A single neuron s axon can transmit information to other cells over long distances (even four feet), while other neurons may transmit information short distances to neighboring cells. Axon endings make contacts on the cell bodies or dendrites of other neurons. The cell body is the part of the cell containing the nucleus. Neurons can have highly branched dendrites, on which axons from other cells can terminate, forming a synapse, where information can be communicated from one cell to the other. 

Figure 28 illustrates the wiring diagram for a simple Wheatstone bridge and thermometer of the four-lead potential terminal type. This method is used mainly for precision measurements in the laboratory. The battery B is first connected to c. As before, when the bridge is balanced we obtain ... 

The measuring cell was made of PTFE and had a four-electrode system connected to its outer cylinder. One pair of electrodes served as terminals for the measuring amplifier, and the outer pair was connected to the current supply through a 10-MO wire-wound resistor (the sample resistance was much less than 10 MQ). The measuring cell was carefully shielded from electric and magnetic fields and from mechanical vibrations. 

To evaluate the sensing characteristics of the conjugated polymer or its nanocomposite using four-probe technique of resistivity measurement, four electrical contacts are made on the layer by copper wire and silver paste. Subsequently the change in voltage between two middle probes on the sample layer is measured in analyte environment and the results are reported in terms of the change in resistivity of the sample at constant current applied trough terminal two probe. The resistivity of the sample layer can be calculated by the equation [12] ... 

The immediate implication of the controlled source concept is that the definition for an electrical circuit given at the beginning of this subsection can be revised to read an electrical circuit or electrical network is an array of interconnected two-terminal elements wired in such a way as to be capable of conducting current. Implicit to this revised definition is the understanding that the two-terminal elements allowed in an electrical circuit are the resistor, the capacitor, the inductor, the voltage source, the current source, and any of the four possible types of dependent generators. 

In this application four individual cells are required to meet the voltage requirement. (The number of cells is, of course, adjustable with minor modification to meet a wide range of voltage needs.) Each cell contains flat circular anodes and cathodes that are separately wired in parallel to achieve the individual cell capacity and plate area needed for a given set of requirements. To fabricate, the components, with intervening separators, are alternately stacked around the cell center tube, after which the parallel connections are made. The cells are individually welded about the inner tube and outer perimeter to form hermetic units ready for series stacking within the battery. Connections from the cells are made to external terminals which are located in the bottom bulkhead of the battery. 

---