接在8V的电源两端使灯泡正常发光,应串联电阻分压;
电阻两端分得的电压为U R=U-U L=8V-6V=2V,
∵
U L
R L =
U R
R ,即
6V
12Ω =
2V
R ,
∴R=4Ω.
故答案为:串;4.