Lilienfeld 1925 Attach Electrical Connections
# Attach Electrical Connections: The Same Foil, Two Different Kinds of Contact
Nine steps have built a device with three electrodes, and this is the first step that reaches outside it — connecting external leads to the two current terminals and to the foil. Two of those three connections are straightforward in a way the third is not. The terminal connections simply need to reach the conducting path Step 9 just confirmed is continuous, without disturbing it. The foil connection is different in kind: everywhere else on its surface, the foil's entire purpose has been staying electrically isolated from the film through Step 3's oxide skin, and this step now has to make a genuine, low-resistance ohmic contact to that same piece of metal — just at a different location, where ohmic contact is exactly what is wanted.
## 1. Attaching a Lead to the Terminals Must Not Undo Step 8's Chemistry
Whatever heat the lead-attachment process applies to a terminal risks disturbing the copper-sulfur compound sitting directly beneath it — the same compound whose conductivity Step 8 showed depends sensitively on sulfur stoichiometry. A temperature high enough to drive further reaction, or to reverse it locally, shifts that stoichiometry away from whatever composition Step 8 actually achieved, right at the one point where the device meets the outside world:
where $T_{\text{decompose}}$ is the temperature above which the compound's own composition starts to change. This is the same shape of constraint this series has already seen govern lead attachment in a completely different device and material system — a process temperature bounded above by whatever the material directly underneath the connection can tolerate — applied here not to avoid remelting a eutectic button, but to avoid quietly rewriting the stoichiometry Step 8 only just finished establishing.
## 2. Real Diagram: Three Connections, Two Different Jobs
## 3. The Terminal Contact and the Foil Contact Mean Opposite Things Electrically
Every lead this step attaches needs a low contact resistance to whatever it is joining, following the same specific-contact-resistivity relationship any metal-to-metal joint obeys:
But what a low $R_{\text{contact}}$ actually accomplishes differs completely between the two kinds of connection this step makes. At a terminal, a low contact resistance simply adds a small, acceptable term in series with Step 9's already-confirmed conducting path — exactly the behavior every lead-attachment step wants. At the foil, a low contact resistance at the lead connection has nothing to do with the foil's working face at all; it exists purely so an external voltage reaches the foil's bulk cleanly, while the oxide skin at the opposite end of that same conductor keeps doing the job Step 3 built it for. The same low-resistance goal, pursued for two entirely different electrical reasons on the same device.
## Real Diagram: Contact Resistance Falls With Area, Independent of Which Job It Does
## Attach Electrical Connections's Place in the Process Lineage
Attach Electrical Connections follows Step 9, Complete the Conducting Path, which confirmed the film this step's terminal leads now connect to is genuinely continuous end to end; it precedes Step 11, Apply the Main DC Bias, which assumes all three connections already exist and simply starts driving current through them. It is the tenth step of this concept's sequence and the first to leave the device's internal construction behind — the point where three electrodes built for three different electrical roles finally become three wires a circuit can actually be built around.